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Battery operated 555 touch circuit

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twingesabit

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I'm working on developing a battery operated touch circuit as shown in the attached circuit. I've been able to prototype the circuit and test it out and everything functions. I moved it over to a breadboard for unit prototyping and everything works.... when it's plugged into a variable DC bench supply. When plugged into a battery it either doesn't work at all or erratically. Contrary to the design, the trigger and reset pins do not need to shunt to source/ground in order to operate the LED. I've found that by just connecting a wire from pin 6 and a wire from pin 2 I can operate the circuit just by touching the stray wire. That's the way I have it set up in the proto box and that's the way it operates on the bench supply. My guess is that grounding is the difference between success and failure. However, given the nature of the design, I'm going to have trouble creating a ground. Can anyone determine a solution that would allow this circuit to work under a 9V battery?

Just a side note, I'm using this to develop a multi touch toy for my daughter, she likes the lights and noises that it produces.
 
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crutschow

Well-Known Member
Most Helpful Member
There is no attached circuit.
 

Boncuk

New Member
Hi twingesabit,

You don't have to create a ground.

Using a battery the negative terminal of the battery is the ground connection for the circuit.

You might as well omit the variable 1M resistor. I saw no effect when simulating the circuit.

Boncuk
 

crutschow

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Most Helpful Member
I believe the floating ground with the battery is the reason for your observed failure, as you suggested. You can test this by connecting the battery common to earth ground.

Since I know of no easy way to ground a portable circuit, I suggest you use a two terminal touch button such as shown in the schematic. You can make one by using gluing two small insulated wires side by side (or use small zip cord type wire) and scraping the insulation off of the exposed side of the wires.
 

twingesabit

New Member
Like I said, the circuit functions under a bench supply. The ground connection has been established via the 0V but operates differently when under a battery as the source. My theory is that the ground connection on the bench supply is picking up the mains hum from the building and providing a more effective connection to the touch plate. However, with the battery connected the circuit is far less sensitive to the mains hum. My question is this.. can the circuit be adjusted to increase sensitivity? Could adding an antenna help? If so, at what point would I had the antenna? Also, the 1M variable has already been removed, I generally make my adjustments with a variable, measure and place a fixed resistor to eliminate unnecessary tweaking.
 

twingesabit

New Member
I was afraid of that. The original design was based solely off the schematic so I had already built up a two-point touch point but discovered (while connected to bench supply) that I was able to trigger and reset with a stand alone point. I was hoping to reduce the size of my build (and parts required lol) by using the stand alones... oh well, the joys of portability. If anyone has a suggestion I would be happy to hear, including any ideas regarding adding an antenna... thanks.
 

crutschow

Well-Known Member
Most Helpful Member
You could try adding an antenna wire to the ground and see if that helps. To make it smaller you might try a ferrite antenna such as used for AM radios.
 

dougy83

Well-Known Member
With the 2-contact switch method, the 1M resistor may need increasing to at least twice the worst case skin resistance. To make it less sensitive to noise, connect a capacitor (10N should be alright) across the contacts.

With a 1 contact method, a large value resistor is used as the pull-up/down. Often a transistor is used to reduce the detection threshold voltage.

In either case, 10M resistors might be more suitable.

Also note that the 555 draws something like 5mA continuously, so take that into account when thinking about battery longevity. The CMOS 555 uses much less current.
 
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