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George

In article ,
says...

Since the relay requires at least 12V, and a fair
amount of current, I used a MAX232 chip to convert the
incoming 6V from the battery pack to a much higher
voltage the relay can use, and a large capacitor to
store that charge between presses. The two-transistor
coil driver was suggested by James in the .design
group, but the resistor values are all my fault.
Thanks to James and all the others there who helped me
figure this out.

If your relay coil resistance is really 100ohm, I don't
see how that MAX232 can supply the needed 120mA. From
the datasheet, the MAX232 can only supply about 10mA
(short circuit), but this is with 1uF caps (not the 10uF
that you use).

What voltage are you really getting across the coil when
it's on?

You are correct that the MAX232 can't supply anywhere near
enough current on a continuous basis. But over time it can
charge up a capacitor with enough juice so that when the
time comes to push the button, the accumulated charge on the
cap can be dumped all at once through the coil, and that's
enough to make it work for a short time.

I use the Tx outputs of the MAX232, one high and one low, to
charge the 220 uF cap, which process takes about three full
seconds to complete after the previous discharge. You can
watch the voltage across the cap ramp up with an analog
voltmeter, just like the books say it should. At that
point, it's charged to about 22V. And during that three
seconds, the current drawn by the MAX232 is about 30ma
maximum, which is pretty much in line with the data sheet.

I have no way to measure how much current the cap discharge
actually produces, but it's enough to pull in the relay and
push the button for a fraction of a second, which is all I
need. So basically, it's really all about doing a slow,
low-current charge of the big cap, followed by a sudden,
hi-current discharge.

By the way, I don't think the 10uF caps make any difference.
I used them because I had them and didn't have four 1uF's.