Advantages of Electronic Touch Switches
Traditional mechanical contacts are prone to wear, leading to durability issues. To address this, electronic ‘touch switches’ have become practical alternatives in various situations. These touch switches utilize the skin’s resistance for switching actions. The circuit design outlined here senses the skin’s resistance and translates it into a useful switching signal. The touch switch contacts can be fashioned from small metal plates, rivets, nails, etc., positioned closely together on a non-conductive surface. This specific circuit employs an LM393 comparator for its operation.
Circuit Configuration: Sensing Skin Resistance
In the idle state, a voltage, equivalent to the power supply voltage, is present via R1 on the non-inverting input of IC1a. By setting the inverting input of IC1a with R2, D3 to D5 at the supply voltage minus 1.8 V. The open-collector output of IC1a, through R3, equals the power supply voltage. This voltage is then inverted by IC1.b. The non-inverting input of IC1.b, determined by the voltage divider R4 and R5, is half the power supply voltage, lower than the voltage on the inverting input. Consequently, the output of IC1.b remains ‘0’ in this state.
Activation through Skin Contact: Sensing Human Touch
When a finger bridges the two touch contacts. The voltage at the non-inverting input decreases sufficiently to trigger the comparator, causing it to toggle states. Human skin’s natural moisture results in a resistance ranging from 1 to 10 MΩ. Enabling this circuit to respond effectively to touch. In environments near mains-connected equipment. It may only require touching the upper contact to activate the switch, given the circuit is properly grounded. In this scenario, the body acts as an antenna, receiving the 50 Hz (or 60 Hz) mains frequency, toggling IC1.a at the same frequency. C1/R3 combination prevents this frequency from reaching IC1b, generating a usable 10-second ‘pulse’ at IC1.b’s output. It’s important to note that even a tiny disturbance, like a fly walking across the touch switch, can generate a switching signal. Consequently, it’s advisable not to control critical systems with this circuit, such as heating systems or garage doors.
Cautionary Considerations: Preventing Interference
To maintain circuit reliability, avoid using overly long wires between the touch contacts and the circuit, minimizing the chances of interference. Additionally, given the circuit’s sensitivity to various disturbances, it’s crucial to exercise caution in its applications. As the power supply voltage is not highly critical, any regulated DC voltage within the range of 6 to 20 V can be employed for the circuit’s operation.