As presented in a previous article, TSC9402 chip produced by Teledyne Semiconductor, may be particularly suitable for implementation, with a low cost frequency voltage converter and voltage-frequency converter.
For this electronic scheme input current is 10 uA, input voltage range is between 10 mV and 10 V, while the output frequency can be set between 10 Hz and 10 kHz.
Circuit has two outputs, one at pin 8 provides a short pulse whose period is proportional to input current, while that of the pin 10 delivers a square wave whose frequency is half the pulse to pin 8.
Using this circuit diagram and few electronic components can be designed a circuit which generate both visual and audible warning. When the CMOS inverters IC1b IC1a forms a bistable open / closed, activated by the touch sensor, and the R1 provides a positive reaction so the circuit will be locked in the state was left.
Bistabil output control IC1c-IC1d astabil which is designed to oscillate at a frequency of about 11 Hz. This frequency is determined by R2 and C1.
When the sensor is touched ON, the LED lights up and buzzer sounds intermittently.