Design Help 2 - Please NO PIC

jamesgangnc · 2009-03-04T14:34:10+00:00

I'm still waiting for my newark order so I went ahead and worked on the 555 solution. Based on the suggestions I got I came up with this. It has a few more components than some of the other solutions but it is working pretty well. A 100uf capacitor is giving me just about exactly 2 minutes of delay before off. I imagine there may be ways to eliminate the front transistor but for me it helped to have each active component doing one task. The transistor on the front deals with my normally closed, open is on, pressure switch. The 555 gives me 2 minutes of delay. And the transistor at the end drives the relay. The release is very clean and consistent. The relay is one of those 12 volt "ice cube" ones and is drawing about 150ma so I suspect I could get rid of the second transistor. The circuit draws about 14ma when "off" and I think I can live with that. I'm still planing to try some of the other suggestions once I get more parts. Thanks to everyone for all the help. I have learned a bit more :-)

Mar 04, 2009 40 Replies

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Hi Jon, I just set this up quickly on the corner of my lab bench. I stuck the capacitor into a white protoboard, attached a 1 G ohm resistor to one leg and measured the voltage from the other side of the resistor to the opposite side of the capacitor with my 10 Meg ohm input impedance voltmeter. This gave me a 1/100 voltage reading and discharged the cap at only a 10-20 nA rate. I applied 20 (something) volts to the cap for a few seconds and then recorded the voltage at different times

Time Voltage (mV)

2:26 213 2:31 202 2:44 195 3:00 190 4:00 185

8:36 AM 156 (So this was the next morning 16 hours later)

I then read the heat idea and taped a 100 ohm aluminum housed power resistor to the top of the cap, tapped a thermal couple to the side, stuck some melamine foam around the outside and top. I also placed a book on top of the foam to try and hold the contraption together. (I don't want you to think this was some precision measurement.) I applied some power to the resistor,

Time Voltage(mV) Temp

10:25 150 54C 11:18 137 61C 11:29 136 61C (I then cranked up the temperature a bit more) 11:40 128 93C 11:45 120 95C 11:52 110 98C Something is beginning to stink I reduced power 12:00 100 95C Heat off, removed the foam. 12:11 98 25C End of the experiment

There was a bit of noise in the measurements as you might expect with the 1G ohm source driving the voltmeter. As I got near it to make a measurement the voltage would jump and then settle back.

The good news is that at room temperature the caps hold their charge for a long time.

George H.