Vactrol Touch Switch

Discussion in 'General Electronics Chat' started by DanRilley, Nov 15, 2009.

1. DanRilley Thread Starter Senior Member

Jan 13, 2008
107
0
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?

2. DanRilley Thread Starter Senior Member

Jan 13, 2008
107
0
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?

3. Wendy Moderator

Mar 24, 2008
20,772
2,540
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.

4. DanRilley Thread Starter Senior Member

Jan 13, 2008
107
0
Cool. Yes it's working so I'm happy about that. I think the current is fine. Now there is a new problem. If the R3 is set to a very low value, like around 2K, the touch switch becomes so sensitive that even getting near it activates the vactrol. In fact, even if I just touch the bottom side of the touch switch without making the body connection from top to bottom this activates it as well. Is there any way to make it only work if a finger is touching both sides of the switch? I though about increasing the value of the pull-up resistor R4, although this works to desensitize the switch, it messes with the voltage going into the vactrol.

Also what's the deal with my body just getting close to it setting it off? If I keep a finger down on the top side of the touch switch it seems to "ground" it, so the proximity affect disappears. But if I am not touching it and just waving my hand over the switch it acts like a theremin. Although cool, this is not what I'm looking for.

Any ideas?

Last edited: Nov 15, 2009