Photovoltaic switch disables unused LEDs

Posted on Mar 20, 2013

In many applications, it`s desirable to disable LEDs used for system verification. Many options are available for the disabling function, including manual SPST (single-pole single-throw) switches, enhancement- and depletion-mode MOSFETs, bipolar-junction transistors, and JFETs. The circuit in Figure 1 automatically disables the LEDs when a mechanical housing encloses the circuit card, thereby preventing you from accidentally leaving the LEDs on to waste power.

The main switch portion of the circuit comprises an amplified photovoltaic cell (photodiode) and a small, n-channel MOSFET. The amplified photodiode signal provides drive to the MOSFET`s gate when enough light is available. Because the photodiode gen

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