Vactrol Touch Switch

DanRilley

Joined Jan 13, 2008
107
Hi I am 4ever a noob.

Here's my question I am trying to make a touch switch that controls an optocoupler. I have it working but I am worried that I may be exceeding the optocoupler's max ratings (40ma). I was wondering if someone could let me know how, with the following circuit, I can make sure that the current feeding into the LED side of the Vactrol does not exceed 40mA.

+V = 4.5V

Schematic:

I really like the results I'm getting from the Vactrol now. Using a resistance range from +V to the Vac of 100 to 22K. What I'm worried about is that 100ohms will result in too high current:

V=IR
4.5 = I*100
I = .045A

How can I get that down to 40mA but maintain the range I am seeing now?

DanRilley

Joined Jan 13, 2008
107
Wait a second .045A is 45mA. I was thinking 450mA. I guess that's fine. I'm an idiot and a genius at the same time and 4ever a noob.

Anyways does the circuit look OK in other respects I hacked it together from several example circuits?

Wendy

Joined Mar 24, 2008
21,840
I'm assuming 4.5V as the power supply, you didn't define that clearly. If Q1/Q2 are fully conducting you will get 1.2 V on the collector. This will put 1.8V on the emitter of Q3. R1/R3 will have 2.7V across them, which sets up a max constant current of 27ma. I don't believe there is a problem.

If it works, it is good enough.