Ask MetaFilter questions tagged with circuit

I need help understanding FETs.

backseatpilot — Tue, 17 Nov 2009 06:18:27 -0800

I need some electronics help - why is this FET not doing what I expect it to? (Please use small words as this mechanical engineer does not often understand the world of electrons.)

I have a circuit that's driving me up a wall! In an effort to build a weather observation station, I have a capacitative humidity sensor that is built into an op-amp integrator circuit. Something similar to this (pdf, see figure 3). I'm using an Arduino to control everything, basically following the same functional block diagram as is in the document - power HI, find the time at V1, time at V2, do some math and figure out the capacitance which is then converted to relative humidity. Then flip the power to LOW and do the same thing in reverse.

The problem, as is common with integrator circuits like this, is that the sensor/cap needs to be discharged occasionally. Based on some other suggestions I found on the internet, I added a JFET in parallel with the sensor and modified my code so that the JFET would close after every hi/low cycle, shorting the sensor and removing the built-up charge.

Unfortunately, it seems that it's in a constant state of short-circuit. If the JFET is in place (and I'm sure that I have the pins where they should be), the integrator circuit does not work. The sensor simply does not build a potential across its terminals, and I'm at a loss why the JFET won't stay in an "open" state.

Is a JFET not the right tool for the job here? Is there some quality of FETs that I'm not understanding? I've done all the most obvious troubleshooting and I'm still at a loss.

Circuit Builder in Minneapolis

ian1977 — Wed, 05 Aug 2009 17:54:42 -0800

Circuit builder in Minneapolis area? I would like to find a circuit builder in the Minneapolis area to build some prototype circuitry for me.

Does anyone have a favorite circuit builder in the Twin Cities (Minneapolis/Saint Paul) area that they would recommend?

Thanks!

Motor needs some drying and tlc

stormygrey — Wed, 27 May 2009 06:36:46 -0800

We need to salvage an the blower motor in the our ac unit. So, our ac wasn't working so well so we decided to clean the coils and such, unfortunately I think we got the motor wet. It will turn on for a while then it flips the breaker. While being 5 seconds or so. What if anything can I do to dry everything out and if I succeed in doing that, will it still work anyhow?

Circuit Drawer

iconjack — Tue, 14 Apr 2009 22:37:04 -0800

Anybody know of any free software that will take my logic equations and make a circuit diagram? As an example, the following describes a circuit with three inputs and three outputs. I'd like to have this automatically drawn up with the usual symbols for and-gates, or-gates, and inverters. Of course the syntax can be anything similar to this.

Inputs X0, X1, X2; outputs Y0, Y1, Y2.

W = ! ( X0∙X1 + X1∙X2 + X2∙X0 )
Z = ! ( X0∙X1∙X2 + (X0 + X1 + X2)∙W )

Y0 = W∙(Z + X1 + X2) + Z∙X1∙X2
Y1 = W∙(Z + X0 + X2) + Z∙X0∙X2
Y2 = W∙(Z + X0 + X1) + Z∙X0∙X1

The circuit, she is broken.

Nonce — Sat, 04 Apr 2009 08:35:23 -0800

How can I test wiring for a car radio? I recently purchased a used car that still has the factory radio in it. The rear speakers do not work. I dismantled a few things and took the radio out, and there were no obvious problems with the connection or radio.

Is there a device I can hook up to the wires at the rear speakers to see if they are part of a complete circuit or have a break in them?

My understanding is that there are three options: a break in the wiring to the rear, an internal failing in the head unit, or both speakers are busted and need to be replaced. I welcome advice on how to effectively test each possible fault.

I don't think using a simple multimeter to test for current would be much help, as I would still be facing the possibility of there not being any signal sent out due to a fault in the head unit. And I would prefer not to solder or crimp wires until I know for sure what the problem is, and am replacing whatever needs to be replaced.

What's the best way for a Crackle Box-type circuit?

itchi23 — Tue, 24 Mar 2009 12:46:07 -0800

What's the best way to make a Crackle Box-type circuit (where the user touches and closes the circuit with their fingers)? I'm starting a project where I'd like to use a printed design as the basis for a Crackle Box-type interface...and I'm trying to find the best way to make the circuit.
The circuit is just going to act as a simple on/off trigger switch (into an Arduino or Make Controller Kit - just in case that effects the process.) FWIW, I have some basic electronics knowledge (ex. - I can muddle through circuit bending, hack little motors to run off batteries, and I've soldered a few small circuit board kits), so I'm using this project as an entrance to switch/trigger building. I'm about 75% sure I can get this switch cooperating w/ my controller board, but I'm soliciting advice on the making/printing of the actual circuit.

Initially, I was thinking of transferring the design via screen printing conductive ink onto a surface, but I'm having a bit of time finding screen printable conductive ink. I also don't know how conductive fingers are, so I'm not sure the right type of conductive material/ink to use. It will be battery-powered (a few double A's or a 9V), so there's no(?!) risk of inadvertently making a PainStation.

I'm also wondering about the material surface the design will be printed on, as I'm sure different surfaces affect the quality of a printed circuit. Would something w/ a high visual reflectivity (glass, polished stone, plexi, etc) hinder or dampen the circuit? This is also why I'm not going for the more traditional PCB-route; even if I did want to get into home-etched PCB's, I'm interested in more experimental/creative surfaces for the circuit to lie on.

Anyone have any suggestions? Thanks!

Radio has no future! - Lord Kelvin

mikw — Fri, 13 Mar 2009 21:21:22 -0800

Where can I buy a discontinued 11C90 650 MHz prescaler? This is for reverse engineering a SR-2020 mystery receiver that a friend of mine has been working on. Any information on this will be very much appreciated. Bonus hand drawn analog circuit diagrams. There's some more numbers associated with the 11C90 however I'm not sure what they mean: U5.75H90. It appears to be discontinued sometime in 2001, however I can only find these for sale in bulk quantities. I'm really just after one or two of these, not a giant stack. The photo set is of a mystery receiver being reverse engineered by a friend of mine. It appears to be of military origin and it and a twin were purchased in Australia at a junk sale. He's been working on it for a number of years as a hobby and has managed to coax quite a bit of life out of it. He'd be extremely interested in any information, no matter how oblique.

Should I buy it now? ... how about now? ... now?

simplethings — Mon, 02 Feb 2009 13:12:33 -0800

Is there a general liquidation timeline I can use to determine when to purchase an item? I'm specifically looking at Circuit City. I was in there last week and saw that a DSLR (the Canon Xsi) I've been eyeing was at this location and was on sale for 10% off. I went again on Thursday and it was down to 20% off. I just came back from checking today and it was still 20% off and in stock. When would be the best time to purchase it? By "best" I mean lowest price but still in stock. Do liquidation sales go on for months? Or will they just keep decreasing prices so that they can sell everything in a few weeks?
I've considered visiting the store every day in hopes I can catch the next big price drop, but, the closest store is not that close.
Do you think people would buy all of the cameras if it became 21% off?

I guess there are a lot of factors that go into this but, any help would be appreciated.

Mic level vs. Line level FIGHT!

knowles — Sun, 25 Jan 2009 07:50:00 -0800

How do I build a microphone preamp for a Macbook Pro? Oh hai. I just recently learned that the 3.5mm input on my MBP (core duo, 2.16ghz, 17in) is not actually a 3.5mm mic in, but a 3.5mm stereo line-in. What's the simplest circuit I can build to remedy this? What about more complex ones?

I bought a Zalman ZM-MIC1 and assumed that it would work out of box, not knowing that I would need some kind of preamp. I have soldering skills and a decent understanding of what electronic components do, how do I get the mic output up to line level for like six bucks at Radio Shack?

If you're so inclined, explain what is making the loudness happen and suggest cool modifications for this circuit as well.

gps module circuit diagram needed

raphael19 — Tue, 05 Aug 2008 13:56:15 -0800

can you help me locate a circuit diagram of a gps module on internet? title says it all.

i googled and searched but could not find any sources. is it top secret or am i not looking close enough? the ones i have found are gps kits with already commercially available modules. but i want the module itself.

Garage circuit breaker problems.

starman — Mon, 14 Jul 2008 07:16:19 -0800

Garage circuit breaker keeps tripping... We had some work done in the backyard (new patio). The workers dug around the cable of wires going to the garage, careful not to damage it. As far as I know, they didn't.

Ever since that first dig, the breaker to the garage consistently trips. Sometimes it's two minutes, sometimes it's six hours.

I looked at the box going out of the house, it looked fine to my untrained eye (wire caps in place, etc.).

It's a 15 amp breaker.

My main question: Would slicing that cable cause a breaker to randomly trip?

Any other theories? Lightning maybe? If the wire is fine, should the breaker just be replaced? If it was the wire would stringing a new one around the new patio be the easiest solution? I'm still talking with the landscapers to fix this, too.

Behold the MT-10,000

tmcw — Mon, 30 Jun 2008 18:48:15 -0800

Messin with electronics.. filter. I'd like to slightly circuit-bend my Casiotone MT-100 - creating a switch that cuts one circuit when power is applied. Okay, so here's the deal. When I play a chord, and switch the Casio Chord mode, the chord is essentially triggered again - the notes start a anew just as I had played them right then.

The plan is pretty simple in my mind. The drums are controlled separately (fairly simple to locate the trimpots for each drum). There's also a LED which blinks each measure. I'd like to pilfer a tiny bit of this voltage in order to possibly run into a transistor that cuts the power to the Casio Chord selector. (Note: the selector is a slide switch).

Okay, so questions: Is a transistor the right thing to use for this purpose (essentially I want an electric switch activated by a current?) What would this effect be called?

Heres a spec on a similar keyboard if that helps
410V
Sound Club Article

BUZZ - I know!!

Jakey — Thu, 26 Jun 2008 17:19:00 -0800

Electronics filter. Game show buzzer+. There are tons of simple game show buzzer layouts on the google. What I can't seem to find is an idiots guide to something that would allow you to take an input more complicated that a simple yes/no. i.e. interrupt + [string]. Any pointers?

A grand for two circuit breakers?

Rendus — Sun, 25 May 2008 12:45:26 -0800

Electrical Estimate Filter: Short version is, is $1100 USD a proper estimate for simply replacing a 125 amp breaker and a 60 amp breaker in a standard residential breaker box? I live in Las Vegas, in a house I rent. Last year, the 125 amp breaker would trip seemingly at random during the summer. The only consistent pattern was that the air conditioner was running, which is almost always the case. This year, the same thing has begun already, despite a decrease in household load. While all of the appliances are electric, we don't run them during the day, so peak load is AC compressor and refrigerator compressor coming on at the same time while 2 computers are running.

The AC unit was inspected by the company that installed it 3 years ago, and they found no problems. The electrician that was hired to come out says the 60 amp breaker for the AC unit will not trip, and the 125 amp breaker also needs to be replaced. However, he then went on to blame the AC unit for tripping things anyway. He seemed reluctant to discuss details with me (as I rent), but the quote he left behind states they will charge $420 to replace the 60 amp breaker, and $680 to replace the 125.

This pricing seems excessive to me, especially if it's done on one service call. I'm hoping for advice on how to continue. It's not my money, in the end, as the landlord should be covering any expenses, but I want to save the landlord any unnecessary expense if possible.

Is Curves that bad? Other options?

LolaGeek — Sun, 20 Apr 2008 10:02:26 -0800

Is Curves International (i.e. the company behind Curves gyms) really that bad? If it is, are there any other gyms that are similar to this chain in terms of target clientele and training plan? I've read the controversies about the CEO of Curves - articles stating that he donates large amounts of money to pro-life/anti-abortion groups; and then rebuttals stating that he gives money to many different organizations, this was just one of them, and he'll reduce the amount. Almost everyone I've talked to about this is pretty anti-Curves.

But, I am getting a little blubbery due to my sedentary job, and there is a Curves in a really convenient location that I pass on my way home from work. I like the fact that they are women-only, and they offer a fast circuit workout.

So is the controversy really true? If so, is there a similar gym I might join that is women-only and offers the same quick type of circuit program? Particularly in the King of Prussia, PA area?

Everything that I build starts to smoke...

jivesoul — Wed, 12 Mar 2008 19:15:08 -0800

I need help designing a simple circuit before I accidentally catch my desk on fire. In order to test some equipment at work, I need to build a circuit that behaves like the following:

1. Tester pushes a push button
2. LED turns on
3. Circuit then supplies a voltage of 25-30 volts for approx. 10 seconds to one output line
4. Circuit then supplies a voltage of 25-30 volts for approx. 10 seconds to another output line
5. LED indicator turns off

Other considerations:
--I need to use this on a few thousand devices, so anything I can do to maximize battery life would be awesome
--Safety. What should I consider in a circuit like this?

I'm trying to use three 9 volt batteries in series as my power supply, but would be open to any suggestions for a better source.

Can anyone point me in the right direction? I've been tinkering with a 555 timer, but I'm struggling with the alternating output lines as well as the correct output voltages.

How does only part of a curcuit go down?

Lokheed — Fri, 26 Oct 2007 00:58:53 -0800

Any electricians out there? How can a household circuit *halfway* stop working? I am confused. Details inside. (Sorry for the length, but I am trying to be as detailed as possible)

In my house, there is a single 15 amp circuit that covers the garage, my home office, and the living room. From the garage, the circuit enters the office, feeds five outlets plus a ceiling light, and then carries on into the living room. More precisely, it enters the office at one outlet, the line carries one direction to another outlet and stops, and carries the other direction to the third outlet which is also switched to the wall switch and ceiling light, and then around to the rest of the outlets and on to the living room.

Last night while working late, the power went out in the office (and not the rest of the house) for a few seconds. After it came back on, I went to turn on the overhead light and immediately the power went out again.

Now here is where it gets weird. The circuit breaker was not tripped when the power went out. There was still power to the garage, and to the first two outlets in the office. From the third outlet (the one switched to the overhead light) and onward all the way through to the living room there was no power. I checked the breaker box, and although it had not tripped I did manually turn the breaker off and then back on. When that one breaker was turned off, there was no power in the garage or in the two outlets in the office. When I turned the breaker back on, power resumed in the garage and the two office outlets, but not in the rest of the office or the living room. After an hour like this, I gave up and went to bed, with the obvious plan to call an electrician in the morning.

In the morning, the power was back on everywhere. I still called the electrician, who recommended that I run a dedicated 20 amp circuit into the office because of all the computer equipment. I will definitely do this. But I still don't understand how *half* of a circuit can go down. If there was a short, shouldn't that by definition take out the entire circuit and trip the breaker? If it was too much load on the circuit, shouldn't something similar have happened? Can anyone explain this to me?

Circuit Simulator in Java

yellowbkpk — Wed, 03 Oct 2007 21:15:19 -0800

Can you help me solve this (not homework) computer science problem? A friend of mine (who is a physics professor) asked me to write a program for him that would implement this:

http://yellowbkpk.com/algebra%20circuits.pdf

To do this, I wrote a Java program with a data model that looks somewhat like this:

Addition, Negation, Reciprocal, Opposite, Input, and Output are all concrete implementations of a "circuit element" abstract class.

A circuit element has an array of inputs, each of which is a reference to another circuit element.

A circuit element has an abstract getValue() method. For example, the Addition circuit element returns input[0].getValue() + input[1].getValue().

The output for the circuit is computed when the Output circuit element calls its input's getValue() (which initiates the recursion).

The problem with this design is that it only works for combinatoric circuits (like the "subtraction" example in the PDF). When I try something like the bottom three examples, I get infinite recursion.

Can you suggest a better way to implement this?

Tell me why I just spent $75.

dhartung — Fri, 21 Sep 2007 07:30:58 -0800

So the electrician said nothing was wrong. But something WAS wrong, only it fixed itself by the time he got there. Help me diagnose this maddening problem our tenant had. So our tenant tripped the main breaker Wednesday night. I reset that and went in the apartment to check the breaker box, and nothing inside was tripped. This can happen, but isn't normally supposed to. In any case, all seemed well, the breaker didn't retrip, and I went to bed.

Thursday afternoon, I was called over because half her appliances were dead. The hot water heater, the HVAC, her electric range, and her fridge were all kaput. Her lights were on, as well as the TV, and most other things in the apartment. NO breakers were tripped. I reset them one by one by the book (off ... pause ... on) and still none of these came on. Even a fluorescent light in the closet that's on the same breaker as the hot water heater worked while the heater itself did not. The HVAC, on a wholly dedicated circuit, did not come on even after resetting. The stove's idiot light was even off, let alone any of the burners. The fridge was the odd man out -- the only appliance on a normal circuit, though it naturally had a dedicated outlet in the kitchen. I tested and reset every GFCI outlet in the apartment (one bathroom, two kitchen) to no effect.

Over the phone the service manager for the top-flight electrician in town (regional contractor with sterling reputation) explained that it could be the breaker was experiencing "loss of phase", i.e. one of the two 120V supply lines that make 240V service possible. This seems very plausible in that the big-ass appliances wouldn't come on, but not the fridge.

Just to check myself I went back to the apartment last night with a pigtail tester and verified that the situation had not changed. Most frustrating, I showed power to the fridge outlet but nothing happening inside.

Despite this sanity check, when the electrician showed up at 7am everything was working again ... except the fridge. He opened up the house panel and tested the points inside, finding nothing amiss. He opened up the main breaker outside and found nothing to concern him there, either. I asked about moisture (we've had some trips that could have been wet-related) and he showed me how there was no evidence of corrosion inside the main breaker box. But since he had verified the panel, the breaker, and even the fridge receptacle, there wasn't anything he could do that wasn't costing us money while stabbing in the dark. So I took it as a costly sanity check.

I'm assuming the fridge is, in fact, dead. It's not especially new and it could easily have kicked the bucket due to a power fluctuation. (But would that kill the light?!) But I can't rest easily knowing I have people living in a place where there's an undiagnosed power glitch.

To inoculate some questions:
* The fridge being plugged in was not a factor. It was plugged in last night and this morning.
* The hot water heater must have kicked in with everything else sometime in the dark of night. It was not all the way hot yet.
* I did NOT unfortunately pigtail test the stove receptacle (last night).
* The tenant added a satellite dish (w/o permission, and I'm pissed about it for other reasons) which is the only recent change to the apartment. That couldn't have done anything ... right?
* No other tenants reported any problems, so it isn't a problem with the building service.
* The interior and exterior electrical equipment is all Square D, and I agree with the electrician that this is a very solid brand. He said loss of phase was possible but very rare, unless you have another brand.

I've read that loss of phase can screw up motors, including compressors. Could that have led to the fridge failure many hours after the circuit was reset? But the fridge being plugged in should not have affected other appliances, unless they too were affected directly by the loss of phase. But why would it kick back in by itself in the middle of the night?!

Summarizing, I don't object to the call as it eased my mind about a couple of different things. But I wish I could have gotten things working last night and avoided it entirely. I did worry that we had to replace a hot water heater AND an HVAC, both relatively new (last 2 years), and that didn't happen, so that's good. The stove belongs to the tenant, and so she can't make a damage claim against us, so that's good. And we have to buy a new fridge, so that's bad. And we don't know why it happened, so maybe the new fridge will get the same treatment at some point?

Did I miss any obvious steps here?

Not That Kind of Transformer!

spacewrench — Mon, 03 Sep 2007 08:13:09 -0800

ElectronicsFilter: How do I drive a step-up transformer to get hundreds of volts out of an AA battery? They're only 1.5v. I have a tiny step-up transformer I salvaged from a disposable-camera flash circuit. I know it worked before I took it apart, but I was reluctant to poke around in there with my meter or 'scope probes because I didn't want to fry anything (or zap myself), so I don't know what the driving frequency/duty cycle is (was). I'm using a 555 oscillator to drive a transistor which pulls current pulses down through the primary, and I'm rectifying the secondary and (theoretically) putting charge into a capacitor, just like the original flash circuit. However, my pulses aren't right, so I'm not getting anything out of the secondary.

What's the easiest way to figure out how to drive the transformer so I can get the highest voltage out of it? I have a couple of meters and an oscilloscope, but no signal generator or advanced tools. I'm hoping to get 80-90v at a few mA to drive nixie tubes.

I (don't) have the power!

designbot — Thu, 30 Aug 2007 12:12:11 -0800

2 outlets & a ceiling light stopped working at the same time. None of the circuit breakers were flipped. What would cause this, and how can I troubleshoot it? I left the house in the morning, and everything was working fine. When I came back a couple of hours later, two of the outlets (well, four outlets in two wallplates) weren't working, and I couldn't turn on the track lighting on my ceiling, which is controlled by a wall switch.

The other outlets in the room work fine. It seems like a circuit was blown or something, but none of the circuit breakers were flipped. I tried flipping them all off and on again for good measure, but it had no effect.

Where else can I look to track this down without involving an expensive electrician? I have some basic equipment, like an LED tester (which confirms there's no power). Changing the lightbulbs didn't help.

Help keep my computer from frying.

BrotherCaine — Tue, 24 Jul 2007 21:39:28 -0800

I need to add grounding to an outlet. How bad an idea is it to chain the ground to the ground of another outlet that is on a different circuit a few feet up from that one? Basically I live in an apartment in California. The kitchen and bathroom outlets are grounded. All other outlets have open ground. No outlets have hot/neutral or hot/ground reversed. On the opposite side of the wall that my computer's UPS is plugged into is the kitchen outlet that is grounded. These outlets are on different circuits. I'm thinking about running a wire between the ground terminals on each outlet as a cheap and dirty fix which I will remove when I move out. All outlets are three prong, none GCFI. I intend to do any work with all circuits in the circuit breaker set to off, and will check for voltage in each outlet before starting. I am obviously not an electrician. There does not appear to be any metal conduit around the wires that could be used as a ground.

I found out about the problem because my UPS said I had a wiring fault, but the computer seems to work fine for now.

Also, the circuit breaker is wired oddly (from my limited perspective) in that you need to turn multiple circuits off to cut power to the bedroom outlets.

Is closed circuit HDTV a possibility?

nickerbocker — Mon, 12 Mar 2007 11:01:57 -0800

Is closed circuit HDTV a possibility? My company has had video upgrades in discussion for a little over a year now and we still haven't found a solution that we are satisfied with. We initially were going towards an all digital camera network run through GigaBitE and Cat6 cable. After looking over some configuration, I have not been all that impressed with the quality or the ease of use.

I have brought up that it is getting to the point that if we are looking at anything new that it should be HD compatible.

From my home use experience I can get HDTV signals through a simple RG59 coax cable that runs from my HDTV (w/ tuner) to my antenna on my roof. I've been very pleased with the quality that comes out of this and am now wondering if implementing something similar for our work camera network is feasible.

My question is, are there any close circuit HDTV setups available on the market today? My vision is to be able to plug in each of our cameras into a encoder/modulator/broadcaster device that is able to set each camera on a digital channel in the VHS band and available to an HDTV tuner on this coax network.

This seems to have 3 big benefits that I can see right now:
1. Our existing camera hardware, even though not HD currently, can be used on this network. Over time we can upgrade cameras to produce true HD video to broadcast on this network.
2. Ease of use for the employees. Everyone knows how to flip through channels on their television and the end result would be a similar interface.
3. I'm assuming that the coax HDTV network can be daisy chained so that the cable goes to each encoder/modulator/broadcast box and out the other end. With the Cat6 network that we were looking at each camera had to have an individual wire going to a centralized gigabit switch. This would mean a lot of wires.

The only downside that I can see to setting things up this way is that the signals will be a one-way thing. W/ the Cat6 network we had the option of pan/tilt/zoom cameras that could be controlled from a monitoring station. This may not be possible with close circuit television.

Thoughts and ideas are really needed badly here. I would also like any links to equipment that has implemented what I have in mind. We may even need a consulting group to bring in on the project so that it is done "right."

Bonus question: what would be a good DVR that has HD tuner capabilities and where video files that are recorded with the DVR are accessible over a computer network? The video files need to be computer accessible as well, as in no nasty DRM on top of the files or a horribly proprietary format. We produce our own video here.

Thanks for all your input!

Circuit bending for dummies

starscream — Thu, 01 Mar 2007 12:26:03 -0800

I have a simple Sony reel-to-reel tape deck from the 1960's that I'd like to install a speed control on. Since I have no electronics skills, can anyone here recommend an ace 'circuit bender' in the Los Angeles area? Thanks :)

Kitchen faucet and disposal just mutinied. Help!

donmayo — Thu, 26 Oct 2006 01:47:20 -0800

My kitchen is systematically going on strike. Last week, my kitchen faucet stopped working. The sprayer thingee still works when the water is on. All other water appliances still work. Just no water from the kitchen sink. The next day, my roommate tells me that the garbage disposal isn't running. Everything else on that circuit is running fine. What to do? How do I fix this? Do I call an electrician, a plumber?