Hello I'm sorry this is my first post but I have wound myself up something chronic.

I am making a novelty clock for my son's birthday in late May 2025 (he will be 25, I am 55, these are the times we live in). I realised it's going to be a little dark inside the clock and had a great idea to have a function that if you touched the handle of the clock (it's like a wide carriage clock design) four LEDs would light up inside for a few seconds.

I designed a circuit. It wasn't easy, I've always used two wire touch switches in the past with no problem (presumably using skin resistance), but finding a simple single wire touch switch (presumably using hand capacitance??) was not easy. In the end I found one using Schmitt triggers and breadboarded it up but frustratingly couldn't get the other Schmitt triggers to make a monostable to keep the light on. It was massively unstable all around so I ended up using the other inverters to get it under control and used a 4001 as a plain Monostable. It wasn't pretty but it sort of worked with a single LED so I made it up on a stripboard and got on with the other parts.

As I was getting further forward with the mechanical parts I thought I would make up the four LEDs using dolls house cable I had to make sure the circuit worked 'in the field'. I had two sets of two white LEDs in parallel on about eight inches of cable. They worked fine triggered off the touch switch.

Then I found a handle I liked, and linked it to the circuit with a solder tag and screw. The LEDs came on on power up, but stayed off. If I detached the handle, the circuit worked. If I kept the handle, but replaced the LEDs with four LEDs pushed into my terminal blocked, the circuit worked. But using the handle and the LEDs on the thin cables, no function. I feel like it's just on at the start and won't re-engage.

I tried using the two gates left on the 4001 as inverters. No change.

I found that the circuit worked if I touched the positive rail. I tried changing the 2N7000 for a BS250, to trigger from the bottom rather than the top. It didn't work. I've ended up just driving it straight from the 4001. No change.

Does anybody have an idea of what I can do now, or another whole circuit?

 

One problem I see is 2N7000 isn't a logic level MOSFET, tomorrow wen I am more aware I'll see what I can do to help, I expect a lot of our other members will step in soon. Throwing a problem like this in front of them is like throwing a steak in front of a pack of starving wolves.

Just curious, why didn't you use a protoboard first?

 

Next dumb question: What does your touch sensor look like (pics would be lovely)?

 

https://forum.allaboutcircuits.com/attachments/20241030_114737-jpg.334729/

 

Use one of these: https://www.ebay.co.uk/itm/204803809364

 

One problem I see is 2N7000 isn't a logic level MOSFET, tomorrow wen I am more aware I'll see what I can do to help, I expect a lot of our other members will step in soon. Throwing a problem like this in front of them is like throwing a steak in front of a pack of starving wolves.

Just curious, why didn't you use a protoboard first?

Thank you for your speedy response. I'm not sure the 2N7000 is the problem as I changed it and then took it out altogether with no change in operation. And I did breadboard (I think this is what I call protoboard) it first, trying several circuits out, and this paltry compromise was the most reliable.

I've attached two photos, one showing the moment I've just touched the bare wire. It works perfectly. The other shows the handle attached, and I'm touching it all I like with no response. I thought the handle might be lacquered so sanded off one edge, but no change. I can touch the solder tag. No response. It's the presence of the handle that is stopping the circuit work.

 

https://forum.allaboutcircuits.com/attachments/20241030_114737-jpg.334729/

Thank you very much. This will definitely be a fall back but I tried a 4013 circuit along the way and couldn't get it to work.

 

Use one of these: https://www.ebay.co.uk/itm/204803809364

Thank you. I've ordered one. Doesn't feel the same, but I really appreciate it!

 

Thank you. I've ordered one. Doesn't feel the same, but I really appreciate it!

Welcome to AAC.

The TTP223 on that module is a very nice little chip. That eBay listing is a bit of price gouging, though. Check out this Amazon (US) listing.

You could buy the IC but it is SMT, so it is probably easier to use the module.

 

Welcome to AAC.

The TTP223 on that module is a very nice little chip. That eBay listing is a bit of price gouging, though. Check out this Amazon (US) listing.

You could buy the IC but it is SMT, so it is probably easier to use the module.

Thank you so much for your reply. The price was cheap enough for me frankly, but your links and product reviews gave me ideas that may help me fiddle with my own circuit. Much appreciated.

 

To clarify terms these are protoboards:

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Think of the as a giant socket you can plug ICs and other components, and wires to make a circuit with the parts are 100% reusable except maybe the wires.

 

To clarify terms these are https://www.amazon.com/ELEGOO-Breadboard-Solderless-Breadboards-Electronics/dp/B0CXF1B6GB/ref=sr_1_3?crid=3H7P5EUXWL97A&dib=eyJ2IjoiMSJ9.X82Ju5tr9ZeC-NuAtMs1h1yOftGKQTolpSIZJSHoP9hdTo5ADAAQTxVQQZgsAhxtwFUmEfLXuXKf2rpdTp8UzdF2fdNCeMG-QM7rmAv8hk9AOcoO-4X-6N25GEiEIyQv8tArdNclyTugRDPjZHe892GRy0PbpwO7tSsnWpgTgNaWKLo2TQhBxzCpPvAFCq9auiOFfO_KnkGvbZE9mQRqayPu24yKe2NUnAEJi0dtHAg.worgt2TV1cBdzLFiRkJAk5jxy03fjis9kFeNKuVg8iQ&dib_tag=se&keywords=solderless+protoboard&qid=1743953637&sprefix=solderless+protoboards,aps,121&sr=8-3']protoboards:[/url]

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Think of the as a giant socket you can plug ICs and other components, and wires to make a circuit with the parts are 100% reusable except maybe the wires.

Yep, that's exactly what I used, thanks.

 

The circuit shown in post #1 depicts a sensor scheme using an oscillator that has the output loaded down by an external touch. And it is followed by a whole lot of external circuitry that rectifies that signal, or the lack of it, and uses that to trigger a flipflop arrangement. So it is really very complicated for such a simple function that it is intended to provide. What I see is that the high frequency signal is rectified and filtered, and then passed thru a string of schmitt triggers to drive a flipflop. I also see a single unused gate, which if the input is not connected will cause the whole device to malfunction.

So there will be some constant power draw from the power supply. In addition, the performance of the circuit is very dependent on the capacitance load connected to the "touch"point. That means that the physical arrangement of every bit of the wiring matters a great deal. And that the physical arrangement is probably the source of your problems.

 

The circuit shown in post #1 depicts a sensor scheme using an oscillator that has the output loaded down by an external touch. And it is followed by a whole lot of external circuitry that rectifies that signal, or the lack of it, and uses that to trigger a flipflop arrangement. So it is really very complicated for such a simple function that it is intended to provide. What I see is that the high frequency signal is rectified and filtered, and then passed thru a string of schmitt triggers to drive a flipflop. I also see a single unused gate, which if the input is not connected will cause the whole device to malfunction.

So there will be some constant power draw from the power supply. In addition, the performance of the circuit is very dependent on the capacitance load connected to the "touch"point. That means that the physical arrangement of every bit of the wiring matters a great deal. And that the physical arrangement is probably the source of your problems.

Thanks

 

The second paragraph, about capacitance to the sense wire, is the warning that attention to that segment is very important. It may require increasing the value of that 22picofarad capacitor a bit. Understand that the capacitor versus the external capacitance to "ground" form a voltage divider.

 

Thanks

 

For anyone who would benefit from closure on a question, the answer to this was that I threw away all of my CMOS chip ideas (and the monostable itself) and just used one of the pack of sensors that AlbertHall recommended. Even so, the handle still loaded it with too much capacitance, but extending the range using a thin wire meant I could have the top of the case as a touch point. Even so, and with testing at every point of the way, when I properly attached the top panel of the case, the handle latched the lights on. I was sort of resigned to that, and hadn't glued the sensor wire in in expectation. So it switches on the side panel.



Thanks for all of your help.