Touch volume control circuit

Touch controls are not only used to switch devices on or off. They can also be used to control different functions. One good example is the TV remote control. If it is very important to keep the activated functions for a long period of time, it is always better to use a digital memory system. However, if small drifts in the control status is acceptable, a simple analog design can be used to memorize the status.

The touch volume controller is one such analog memory touch control switch. The main function centers mostly on the IC1. It is an opamp configured as an integrator with a high impedance input. If sensor 1 is touched, the capacitor C2 charges through the skin resistance and voltage at the output of IC1 decreases linearly until it reaches zero volt. Touching the other sensor (sensor 2) will produce the opposite result: the voltage at the pin 6 of IC1 will increase linearly until it reaches the power supply level. The special function of this touch volume control circuit is that after moving your finger away from the sensor(s), the output voltage of IC1 stays at that level.
This voltage value is memorized by C2. This analog memory however has a problem in long time periods: The voltage value drifts away by 2 % per hour due to the unavoidable current leak in the capacitor. To improve this situation, it is highly recommended to put this circuit in a moisture proof box.
This touch volume controller circuit has a wide application range. It can be used in devices where a potentiometer can be controlled through voltage levels. The touch sensors can alse be replaced with conventional push button switches. The capactiors C1 and C4 are very important in the circuit: they prevent the IC1 from oscillating. Simultaneously closing both switches will not damage the circuit.

Touch volume control PCB layout

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