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18 January 2011
Author: Giorgos Lazaridis
DIY AC Hum Touch Button

An AC-Hum touch sensor assembled on a breadboard for test

An AC Hum touch sensor is a very special technique, rarely used for switching applications, because it has a great disadvantage. In order to operate normally, an active AC power line has to be near by. More info about this type of touch sensor, along with other types (including the capacitance method), can be found in the corresponding theory page, how the touch buttons work..

AC Hum touch sensor with momentary make action (Touch button)

If you have already read the corresponding theory about touch sensors, you will not be surprised by the simplicity of this circuit:

As you know, the human body acts as an antenna, which induces the 50 or 60 Hz from near by power lines. When the electrode is touched, the AC signal is transferred through the protective R1 resistor to the base of the darlington pair transistor amplifier. A large electrolytic capacitor connected at the output of this transistor (C1) will smooth the AC signal, same like the smoothing capacitors of a power supply do. The next stage is the switching transistor. I chose to use a PNP type, because the first darlington transistor is connected as an inverting amplifier (worked much better). With this PNP, when the electrode is touched, the load will be activated. Use a NPN to invert operation (for NPN type, emitter and collector are inverted!).

The load is connected to E1 and E2, with E1 being the positive. I should have mark them with L instead, but you get the point. The current limiting resistor R5, must be selected according to the current and the T2 transistor specifications. The one that i use can handle up to 800mA. In case that you connect a relay, this resistor can be totally omitted. Pretty much the same rules are applied, as in the previously-presented circuit (Resistance Touch Button)

 The electrode is released The electrode is touched

Bill Of Materials
 Resistors R1 Resistor 4.7 KOhm 1/4 Watt 5% Carbon Film R2 Resistor 470 KOhm 1/4 Watt 5% Carbon Film R3 Resistor 22 KOhm 1/4 Watt 5% Carbon Film R4 Resistor 100 KOhm 1/4 Watt 5% Carbon Film R5 Selected according to your load Capacitors C1 Electrolytic Capacitor 1 uF 50 Volts Transistors T1 BC517 NPN Darlington Transistor T2 BC327 Switching and Amplifier Applications PNP Epitaxial Silicon Transistor

AC Hum touch sensor with toggle action (Touch Switch)

The previous circuit acts as a momentary touch button. To convert it into an on-off button, i added a Flip-Flop connected as toggle F-F, exactly as i did with the previous circuit (Resistance Touch Button)

The operation is explained in details in the previous circuit, so i will not go through it again. What you need to have in mind, is that, an AC hum touch sensor, is a very special touch sensing method, most of the time not proper for switching applications. I encourage you to read the theory to learn more about it.

 The output is OFF The output is ON

Bill Of Materials
 Resistors R1 Resistor 4.7 KOhm 1/4 Watt 5% Carbon Film R2 Resistor 470 KOhm 1/4 Watt 5% Carbon Film R3 Resistor 22 KOhm 1/4 Watt 5% Carbon Film R4 Resistor 100 KOhm 1/4 Watt 5% Carbon Film R5 Resistor 220 Ohm 1/4 Watt 5% Carbon Film R6 Resistor 220 KOhm 1/4 Watt 5% Carbon Film R7 Resistor 1 KOhm 1/4 Watt 5% Carbon Film R8 Resistor 22 KOhm 1/4 Watt 5% Carbon Film R9 Selected according to your load Capacitors C1 Ceramic Capacitor 0.1 uF 50 Volts C2 Electrolytic Capacitor 4.7 uF 50 Volts C3 Electrolytic Capacitor 1 uF 50 Volts Transistors T1 BC517 NPN Darlington Transistor T2 BC327 Switching and Amplifier Applications PNP Epitaxial Silicon Transistor T3 2N2222 Switching NPN Transistor Integrated Circuits IC1 CD 4013 Dual D-Type Flip-Flop