CR1, CR2, CR3, and CR4 form a bridge circuit with the SCR across the dc legs. With light on the photoconductor PCI, CI charges through Rl to about 150 Vdc. The resistance of PCI is low when illuminated, so very little voltage appears across it or C2. At about 90 volts CI starts discharging through Rl and the SCR, but the SCR cannot turn off until CI is almost completely discharged. When the SCR turns off during the interval line voltage is near zero, the full supply voltage again appears across the bridge, and Cl charges again to a high voltage. The voltage on C2 also starts rising until the neon lamp fires and the cycle repeats.
An alternative remote control can be made by adding a second neon lamp, N2, and masking the photocell so it sees only N2. A very sensitive remote control is thus obtained that is completely isolated from the load circuit. For low-voltage remote control a flashlight lamp may be used instead of N2 and operated at about Vi its normal voltage thus giving exceptionally long life. Performance of the photoelectric control may be inverted (flash when the photoconductor is illuminated) by interchanging PCI, and R2. Sensitivity in either the normal or inverted modes can be decreased by partially masking PCI, and can be increased by increasing resistor R2 to about 470 K. To increase on time, increase Cl; to increase off time, increase R3.