Touch Activated Light Circuit: MOSFET vs. Transistor Design

Description

The circuits presented below activate a 20 watt bulb upon contact and with a skin resistance approximately equal to 2 Megs. The circuit on the left utilizes a power MOSFET that becomes conductive when the voltage between the source and gate reaches about 6 volts. The gate of the MOSFET doesn't consume any current, therefore the voltage on its gate will be half the supply voltage, namely 6 volts, when the resistance across the touch terminals matches the fixed resistance (2 Megs) between the source and gate.

The circuit on the right employs three bipolar transistors to achieve the same outcome, with the touch contact referenced to the negative or ground end of the power supply. As a bipolar transistor consumes current and the current amplification factor is typically less than 200, three transistors are necessary to escalate the microamp current level through the touch contacts to the several amps required by the bulb. For increased current, the bulb could be swapped with a 12 volt relay and a diode across the coil.

Bipolar Junction Transistors (BJTs)

Bipolar Junction Transistors, often abbreviated as BJTs, are semiconductor devices that control current flow based on the voltage applied to their base terminal. They are fundamental components in many electronic circuits, including those used in this collection. A typical bipolar transistor consists of three terminals: the Base, the Collector, and the Emitter. The transistor operates in active mode when a small current applied to the base controls a larger current flowing between the collector and emitter. These transistors are commonly used for amplification and switching applications. The current gain (β or hFE) of a BJT represents the ratio of collector current to base current, and values typically range from 200 to 1000, depending on the specific device. The base current controls the larger current flowing from the collector to the emitter, allowing the transistor to switch or amplify signals. They are frequently employed in circuits needing precise control of current flow and signal amplification.

Circuit diagram