Timer Troubles

July 6, 2007 8:53 PM

I'm making a timer for skateboard racing and need some help. Does anyone have some advice on wiring simple circuits to lend?

THANKS!
The timer I'm making is connected to two pressure sensitive tape switches for the start and finish. The problem I'm having is that at very slow speeds the pressure tape records both front and back wheels separately. At high speeds the connection time in the tape seems too brief to be recorded by the timer.
Is there simple (ie. cheap radioshack item) capacitor or voltage regulator-like circuit that could be added to my tape switches that will register a connection and keep the gate closed for a period of time to ensure the timer receives the signals for start and finish?

Again, thanks.

posted by infomaniac to sports, hobbies, & recreation (8 comments total)

What does the timer consist of? If it's a microcontroller, search for 'debounce switch' and adjust the time to whatever amount it takes the second set of wheels to cross plus a bit. A cheap hack, but it works.
posted by IronLizard at 9:58 PM on July 6, 2007

The timer is - a timer. http://www.technika.com/Sper/s810032.htm

I don't think it was sensitive enough to register the quick "spikes" when the tapeswitches are rolled over too quickly. Just adding voltage (9v battery on top of the AA one in the timer)didn't seem to help. I'll look up debounce switches- sounds good. Do they hold the circuit closed if they detect such spikes?
posted by infomaniac at 10:16 PM on July 6, 2007

The simplest thing you will find at Radio Shack is a TLC555 timer chip. This will operate with a single AA battery. You can configure the chip to generate a single pulse of whatever duration you need when triggered. It will ignore subsequent triggers for the duration of the pulse. Google up "555 timer circuits" and look for a "monostable one-shot". The input to the circuit is a low going pulse and the output is a high going pulse.

You then want to invert the output to generate a low going pulse which you can do with a transistor. Connect the base of the transistor to the output of the 555 with a 10K resistor. Put one of these circuits on each tape switch and wire the two collectors together in an "OR" configuration, then connect to the start/stop input. Connect the emitters together and connect to the neutral/ground lead.

You can lengthen the pulse output of the 555 by selecting resistor and capacitor values to ignore the trigger from the second wheels.
posted by JackFlash at 12:01 AM on July 7, 2007

Ok, you're using a prebuilt unit. Well, short of taking it apart try using something wider as the switch (a bit more than the length of the skateboard) this will solve both problems at once. I'm thinking aluminum foil creatively put under a plastic sheet with a divider but you'll have to work out the details.
posted by IronLizard at 12:29 AM on July 7, 2007

You could ditch the tape switches entirely and use an interrupted light beam instead. Put a laser pointer on one side of the track, shining onto a phototransistor in the back of a deepish black tube on the other side. Not only could you put this at a suitable height to be triggered by the skater's body instead of the wheels (if you wanted), but it wouldn't wear out or ruckle up and trip skaters over and you could use it on any width of track.

Here's an explanation of the pinout of the 555 timer chip, and here's how to wire it as a monostable timer. For your application, you'd substitute the phototransistor for R2, and a 5k resistor for the pushbutton, effectively feeding the Trigger pin on the 555 from the output of the common-collector phototransistor amplifier shown as figure 2 in this application note.

As long as the laser beam was shining down the tube and illuminating the phototransistor, the 555's trigger pin would see a voltage close to Vcc, and the output would stay low. The brief pulse of shadow caused by a skater interrupting the beam would cause a brief low-going signal to the trigger pin, which would start the 555's timing cycle. The 555's output, on pin 3, would rise to Vcc and stay there for the duration of that timing cycle.

If you used a 10k resistor for R1, and a 0.01uF capacitor for C, the 555's output pulse would be about a tenth of a second wide, which should be good enough for just about any timer designed for manual pushbutton inputs.

If your timer wants two wires connected to trigger it, rather than a voltage pulse, you could use a transistor (a BC338 would do fine) to adapt the 555's output. Find which of the timer's input wires is positive and which is negative, using a multimeter; connect the collector of the transistor to the positive wire, the emitter to the negative wire; connect the output of the 555 to one end of a 1k resistor, and the other end of that to the base of the transistor; and connect the GND point on the 555 circuit to the negative wire.

If the timer has a single start/stop input, rather than one for start and another for stop, you could perhaps get away with using a single sensor, and use mirrors to arrange for the same laser beam to be interrupted at the start and finish lines.
posted by flabdablet at 2:44 AM on July 7, 2007

WOW THANKS GUYS! (gals?)

I know someone who built a LED based timer like you describe Flab, but since I already spent the $ on the tape switches, I'm gonna try to make them work. Lengthening the pulses with the 555 timer chip looks to be my answer.
I'm off to radioshack...

Since I'm not really sure how to read the diagrams properly, If I mistakenly stumble onto a superconductor or cold fusion device I'll be sure to post here first. ;o) Wish me luck!
posted by infomaniac at 10:08 AM on July 7, 2007

http://www.ibiblio.org/obp/electricCircuits/Ref/REF_9.html
posted by IronLizard at 12:45 PM on July 7, 2007

Most of what I described will work just as well for the tape switches as it would for a phototransistor sensor. Use the 555 monostable circuit exactly as shown, with a 2.2k resistor for R2 and the tape switch substituted for the pushbutton. Also, wire a 0.01uF capacitor straight across the points where the wires from the tape switch connect to your 555 board, to get rid of noise pickup that could cause the 555 to trigger spontaneously.

Regardless of whether you use a tape switch or a photosensor, the 555 circuit will run fine off a 9V battery. However, since your interval timer runs off a single 1.5V AA cell, you probably will want to use the BC338 transistor between the 555 and the timer's input. I can't imagine a 1.5V-based timer is going to react well to having a 9V signal injected into its guts from a 555.

Post back once you've built it and let us know how it goes!
posted by flabdablet at 6:29 PM on July 7, 2007