touch activated light

Description: The circuit activates a 20-watt lamp when the contacts are touched, provided that the skin resistance is approximately 2 Megohms or lower. The circuit on the left employs a power MOSFET that turns on when the voltage between the source and gate reaches around 6 volts. The gate of the MOSFET does not draw any current, allowing the voltage on the gate to be half of the supply voltage, or 6 volts, when the resistance across the touch contacts equals the fixed resistance of 2 Megohms between the source and gate.

The described circuit utilizes a power MOSFET as a switch to control the operation of a 20-watt lamp based on touch sensitivity. The primary operation hinges on the interaction between the skin resistance of the user and a fixed resistor, which together form a voltage divider. When a person touches the contacts, the effective resistance drops to approximately 2 Megohms or less, allowing sufficient current to flow through the gate-source junction of the MOSFET.

In this configuration, the MOSFET is designed to turn on when the gate-source voltage (Vgs) exceeds the threshold voltage, typically around 2-4 volts for most power MOSFETs, with a nominal operating point set at 6 volts in this case. The gate is connected to a resistive divider formed by the touch contacts and a fixed resistor, which ensures that the gate voltage reaches the required level to activate the MOSFET without drawing significant current from the source.

As the skin resistance varies, the voltage at the gate adjusts accordingly, maintaining the MOSFET in an 'on' state as long as the skin resistance remains below the threshold. This design allows for a responsive and energy-efficient touch-activated lighting solution. The 20-watt lamp, being a resistive load, will illuminate when the MOSFET is activated, providing visual feedback that the circuit is functioning as intended.

Careful consideration must be given to the specifications of the power MOSFET selected for this application, including its maximum drain current, voltage ratings, and thermal characteristics, to ensure reliable operation under the expected conditions. Additionally, the circuit may benefit from protective components such as diodes to prevent back EMF if inductive loads are introduced in future modifications.The circuits light a 20 watt lamp when the contacts are touched and the skin resistance is about 2 Megs or less. The circuit on the left uses a power MOSFET which turns on when the voltage between the source and gate is around 6 volts.

The gate of the MOSFET draws no current so the voltage on the gate will be half the supply voltage or 6 volts when the resistance across the touch contacts is equal to the fixed resistance (2 Megs) between the source and gate..