Human Electrical Current Circuit Touching Hands Transistor NPN

Thread Starter

isagollo

Joined Feb 20, 2021
22
My project purpose is that when you hold hands with a friend the passage of electric current through a circuit is enough to turn a buzzer or the ISD1820 voice recorder speaker. My first idea it to use the circuit below as reference, but I have no idea which transistor I should use to have current enough to turn for example a 3W-5V speaker. Could anyone give me hand?:)
 

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dl324

Joined Mar 30, 2015
16,688
My first idea it to use the circuit below as reference, but I have no idea which transistor I should use
I'd use a logic level N channel MOSFET. It will turn on with much less current going through body parts. You'll need a resistor from the gate to source. I'd try 100k-1M for starters.
 

Thread Starter

isagollo

Joined Feb 20, 2021
22
I'm trying to draw a schematic for this circuit, could you help me out? Where should I place the power supply in the mosfet? And the idea is that only when both friends hold hands together that the circuit turns it on. Where the 2nd touch hand point should be?
 

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Thread Starter

isagollo

Joined Feb 20, 2021
22
What are you trying to do with the speaker? Having the MOSFET switch it like the LED in your first schematic could damage a 3W speaker.
Power, as you showed in your first schematic.
- I'm using the VOICE RECORDER AND PLAYBACK (ISD1820) MODULE with PAM8403 AUDIO AMPLIFIER MODULE BOARD WITH POTENTIOMETER SO THAT I CAN USE A 3W(5V 4Ω) SPEAKER.
- The idea of my circuit is that the 1st person hand is touching a contact point in the circuit and the 2nd person hand is touching another contacting point in the circuit. When they hold their other hands together, the module plays the recorded audio. The audio will only stop playing when they let go of their hands.
 

dl324

Joined Mar 30, 2015
16,688
Play mode is activated by applying power to the PLAYL pin.
1697557981603.png
You need to use a P channel MOSFET:
1697557813115.png

You must use a logic level MOSFET because Vcc is 3.3V.
 

MisterBill2

Joined Jan 23, 2018
17,814
Quite an interesting and unusual project, AND a very well stated goal, which is always handy.. This will be a bit of an interesting challenge because you have, between the two touch points, five unknown resistances: two hand to touch point resistances, two internal person resistances, and one hand to hand resistance. What makes it a bigger challenge is that I think there is a low voltage source and only a very small current is allowed.
Of course there is an alternative, which is to start with a variation of "The Tickle Stick" circuit, which was in some magazine back in the early 1960's era. That could provide a voltage higher than the battery voltage, with the second benefit being that it can be an audio frequency so that it can be boosted with an audio amplifier.
I don't recall much about the tickle-stick except that it used one "D" battery and a small audio transformer. An audio amplifier with no response below 200Hz would avoid 60 or 50 Hz pickup from surrounding mains wiring.
I think that makes the project simpler. Maybe??
 

MisterBill2

Joined Jan 23, 2018
17,814
I think that it makes the task simpler than designing an inexpensive, stable, very high gain DC amplifier that will switch an oscillator on and off. Others may not agree, and some folks here may have no difficulty creating a very stable DC amplifier with a gain of a few thousand and also a very high input resistance, that is very cheap. I am not one of those folks.
 

Sensacell

Joined Jun 19, 2012
3,400
A small detail to consider- if you power this with a mains power supply, you may find that AC leakage currents from the power supply cause erratic operation of the touch circuit.
Stick with batteries and you will have no problems.
 

MisterBill2

Joined Jan 23, 2018
17,814
Definitely go with battery power!!
Notonly would mains power be inconvenient, it would certainly add a serious hazard of shocking, so that the safety requirements would be vastly increased.
This IS a device INTENDED for human electrical connection, after all!!!!!
 
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