Touch Switch Expansion

[AlexHenighan]

Hi, my first time, so please be gentle

I have created a touch switch, using a 555 IC. I have it timed using a variable resistor and a capacitor. All is good, except, I want to be able to be able to use the same touch plate to toggle the circuit on and off..so I can touch and leave it to time out, or touch it again to turn on /off.

I have used a 2n2222 as part of the trigger input. The collector voltage goes from 10v to 7v (measured on a basic multimeter). This is enough to trigger the circuit (Pin2 on the 555). I have tried to adjust the bias resistor on the collector (attached to 12v through a 5Mohm variable resistor) and can reduce the voltage to 8v and 5v respectively when the resistor is at 5Mohm. I tried to up the 5M to a 10M resistor, but the circuit stopped working (although I may have popped the transistor!).

Anyway, to get the logic to allow me to automate a reset (Pin 4 goes low), my solution was to use a 4011 Quad nand IC. I created a logic circuit that gave me an output that would hold the RESET Pin on the 555 HIGH unless the touch plate was touched.

Trouble is....the collector output is at best 8V reducing to 5v when the base is touched and this is not enough to trigger the NAND gate.

What I am trying to do is to take the 8v as a high (easy), but to take the 5V as a LOW (STUCK!!!)....
I thought about using a couple of resistors to reduce the voltage, but I think I need at least 5v difference (high to low)...and also worried that this solution may not be reliable.

So, what I need is some help!! Is there a way to take the 8v to 5v output and easily transform it to 8v and 0v? (enough to trigger the NAND gate).

Happy to show a picture of my circuit so far if that helps.

Any help would be appreciated.

Alex

[pebe]

Yes, a circuit would help.
Do you just want to use the 555 as a switch - rather than as a timer?

[AlexHenighan]

Hi,
I want to have the timer as well as the ability to toggle on/off, using the same touch pad. I will scan the circuit in a little later on.
Many thanks
Alex

[AlexHenighan]

Current Circuit layout (not quite working)

CircuitDiagram.JPG (49.61 KiB) Viewed 23773 times

Hopefully you can see this file. (Again, please don't laugh!! I have not touched an electric circuit in 20years+ and was never talented then )

Many thanks for any help that you can offer.

Regards
Alex

[pebe]

I can give you a tested circuit for a 555 timer with single push-button set or reset. I could also suggest a means of operating it from a touch pad, but I have not tested it and it may need a bit of tweaking. Just say if you want it.

[AlexHenighan]

Hi,
I am happy to receive any support in getting the circuit working, even if it is starting again with something better.
Thanks
Alex

[pebe]

I think this circuit may do what you want.

Fig 1 is a slight variation on a circuit I did some time ago. At startup, the 555 may be triggered because C1 is discharged, and pin2 and pin6 are inactive – hence its state cannot be determined. Assume it has triggered and the pulse has ended. It cannot repeat because pin2 is high.

Here is how it works. The output is low and C2 is discharged, so pressing PB1 will take pin2 low via D2 for long enough to trigger the timer (D1 and D2 are 1N4148 or similar). When PB1 is released C2 will charge through R3 until it is the same voltage as pin3 (high). If nothing more is done with PB1 then the pulse will be its normal length and the output will go low until it is triggered again.

But if PB1 is pressed again during the pulse, the charged C2 will put a positive pulse via D1 on to pin6, causing the timer to reset.

Fig 2 shows a method (untested) to enable a touch plate to simulate PB1. IC1 and IC2 are two sections of a 4066 quad analogue switch (I haven’t shown connections to their supply pins). When IC2 control pin is low the switch is open, but when the control pin goes high there is a connection between IN and OUT pins. R2 presents a high resistance load to the touch plate, and R3 prevents static damage to the IC.

As R1 is adjusted the OUT of IC1 will change between low and high. Set it to a point where OUT is just low, and touching the plate will take it high. Then when the plate is touched C1 will charge with mains hum or noise, raising the control pin level so that ‘A’ switches to ‘B’. A and B now connect to those points in Fig 1 in place of the push button. You may need to increase the value of R4/C1 time constant.

I hope it works OK for you.

(Edit 31.10.13 : Changes to inputs of Fig 1)

[AlexHenighan]

Thank you very much. I will rig up them up (going on holiday now for a couple of weeks, so will be when I return and get the parts) and see how well it works - I will keep you posted. Looking forward to getting this working - and I am clearly a long way below 'good' on my electronic theory, so was definitely in need of assistance - I shall stick to the day job for now

Again, really appreciate your help.
Best Regards
Alex

[AlexHenighan]

Hi,

I have just completed the first circuit and it works exactly as I hoped it would. There are a couple of slight issues - the sensitivity is very high, which results in the toggle function not working smoothly (quite tricky to touch the wire without turning it on/off/on/off/on) If I hold my finger on the plate, you can see the output toggling very rapidly.

I increased the capacitor on ctrl line into IC2, as I thought that it is discharging a little too quickly (from 10nF to 100nF), but little difference. Any ideas?

Also, the duration of the timer is now static. Presently I have R1 as a variable (5M) resistor (have adjusted it through its range and it makes little difference). I think that I will need to change either the 1K or 100K resistors on pins 6 & 7 ? I am looking to get timing up to around 5mins. There is currently a 100uF capacitor in C1.

Thanks again for all your help.

Regards
Alex

[pebe]

Hi Alex,
It looks there are two separate problems. Let’s deal with the timer first.

When I looked again at the circuit I realized that D1 and D2 are always conducting, and there will always be a minimum 200K charging resistance. Sorry about that. Change it to this new circuit where the pot, R1, is in series with R3. Then try this. With the circuit of Fig2 removed, put a short between A and B and ensure the timer starts (pin3 high). Remove the short then put it on again to ensure the timer stops (pin3 low). If those tests are OK then R1 should change the timing OK. Let me know if that is not what is happening.

Regarding the values of R2 and R4. When the timer resets, an internal transistor switches pin7 to ground to discharge C1. R2 is only there to limit the discharge current through that transistor. R4 passes no current and is only there to present an impedance between pin6 and C1, so that pin6 can be pulled high. Neither of those resistors will affect the timing.

If we can get that right we can proceed to the touch switch.

BTW, with large values of C1 and R1, the leakage current in C1 may affect timing. The leakage current will try to discharge C1 as current through R1 is trying to charge it, so times may be longer than calculated.

[AlexHenighan]

ok, will try this out and let you know. And thanks for taking the time to describe 'why ' and 'how'.
Regards
Alex

[AlexHenighan]

I have given this a whirl, with following results.

With the variable resistor (R1) set low, the circuit works perfectly. Timing is short and the circuit toggles on and off when shorting points A & B.

However, as soon as I increase R1 above about 90-100K, the circuit continues to work, (i.e. toggles on/off), however the output of Pin 3 stays high .. well actually, the circuit resets itself and starts to timing again. I know that as you can see the voltage blip on the output and can see Pin2 and Pin6 ramping up during the timed period, then dropping low momentarily before climbing again.

The longest I can get the circuit working 'normally' is for about 20-30 seconds.

In normal operation, the lowest voltage I can get the circuit to work is with 5v on Pin 2 (this increases to 8.4v during the timed cycle).
With this setting, Pin 6 sits at around 4v (I am a little confused as I expected it to be lower due to the 2 diodes (0.6v each?). Anyway, Pin 6 runs from 4v to 7.4v during the timed high on Pin 3.
Pin 7 goes from 0v to about 5.9v during the timed cycle.

I have checked the diodes - they both look ok.

I did give changing R4 a go - Increasing to 540K and managed to get it running to over a minute. I then increased to 790K and had it running to about 1 Min 20...I found I had to adjust R1 to keep Pin 2 to about 5.5v else it became too sensitive. I don't think this approach is correct, but thought it worthy of letting you know.

Please let me know if there is anything you need to know.

[pebe]

Yes, with a high value R1, the discharge can take pin2 below the trigger voltage and it will repeat.

The solution is quite simple, but I'll breadboard it first to be sure it will work OK.

[casdsys]

How about using a Qtouch device. It'll be alot easier to contruct and uses very little components.
Also do a search for Qprox. This will give you multiple switches.

[AlexHenighan]

Hello Casdsys,

Many thanks for the suggestion. I did come across their website before I posted on this forum, but had not seen the specific IC's (only saw the touch screen components, which are not quite what I am looking for). So I ended up going down the 555 route as there was much more information available.

I will bear them in mind as my project progresses - At the moment, with the help that I am getting on this site (thank you), a working prototype is not far off (and I am emotionally invested in this solution now! ). I think I may well experiment further down the line though.

Best Regards
Alex