Digital Electronics: Keeping a logic level high or low for one minute

March 15, 2013 6:13 AM

I want to use a timer (minimum on time is one minute) to turn on my coffee maker. The ON-OFF button on the coffee machine is a momentary contact push button which is read by a logic board. Can I use the (isolated) relay contacts on my timer to close that push button for one minute, the minimum on-time of my timer?

The coffee maker is a Starbucks (Saeco) Sirena, and I need it to pre-heat its little boiler for about 20 minutes before I make coffee. I have a timer (clock) that I have modified to close a pair of relay contacts at the set time. (These contacts are isolated from the 110 VAC line.) The minimum on time for the relay contacts is one minute. (I prefer not to build a 555 timer circuit as a one-shot for a shorter duration). I am quite sure that the power button on the Sirena simply sends a logic high or low to the circuit board (I have the wiring diagram). I have tried holding the power button pressed for over a minute, and the Sirena simply went on, as if I had done a momentary press and hung around for the remainder of the minute.

I want to use the relay contacts (wired across the power pushbutton -- which is a logic controller, not a power controller) to simulate a press. It will either force the logic level to high or ground (depending upon how the Sirena is currently wired).

I think that it is OK to hold that logic level for the minute. What do you think?

posted by mbarryf to Technology (6 answers total)

Best answer: If it works when you do it by hand, you'll probably be fine.

If you want the "keypress" to run for less time just to be on the safe side, you might wire a sizeable capacitor in series with the positive leg of your relay. Your timer will fire and you'll get flow across the capacitor to engage the relay, but as the capacitor charges up it will eventually stop passing current and the relay will dis-engage. You'd have to experiment with the right size capacitor to use. You might also put a fairly large resistor in series with the capacitor so it can bleed off its charge when the timer de energizes.
posted by gjc at 6:53 AM on March 15, 2013

Response by poster: gjc: actually before I realized that the coffee maker didn't mind the long press, I was going to do exactly what you suggested with the capacitor. I still might give it a try. Thanks.
posted by mbarryf at 6:55 AM on March 15, 2013

If you verified the reaction of your coffeemaker to an extended keypress, there's no reason to do anything other than what you plan. It's a brute force approach and not that elegant, but plenty serviceable.

Usually, stuff like this is best done by just trying it out.
posted by FauxScot at 8:58 AM on March 15, 2013

Response by poster: gjc: I think you mean to use a large resistor in parallel with the capacitor to bleed its charge. (Not 'in series'.)

I have 44 micro-farads (200V but the relay is 24 Volt) with a 2.2 Meg-Ohm resistor across the cap. The pulse seems to be OK, but I'll have to wait to check it with the coffee machine.
posted by mbarryf at 9:17 AM on March 15, 2013

Can you just buy a cheap appliance timer and plug it inline with the power cord?
posted by wenestvedt at 10:09 AM on March 15, 2013

Response by poster: wenestvedt: no. In addition to the presence of electrical power, the device must have the power on-off button pressed. I've used simple electrical timers for non-logic-controlled devices, but it won't work for this coffee maker (I just tried).
posted by mbarryf at 12:58 PM on March 15, 2013