Monitor Lamp Driver.

TM 11-5895-1141-34

the 24 volt supply through R10. The lamp driver transistor Q2, is supplied with just enough base current in the absence of

a signal from T4 to bring it up to, but not over, the threshold of conduction. Lamp DS1 thus remains dark. When the

signal level at the input of T4 reaches or exceeds -30 dBm, however, Q1 is driven out of saturation during the negative

half-cycles. The voltage variations appearing at the collection of Q1 are detected by the voltage-doubler circuit consisting

of C3, C8, CR3 and CR4. This delivers additional current to the base of Q2, causing the lamp to glow perceptibly.

Increased signal levels drive Q2 toward saturation, so that signal levels of -10 dBm cause the lamp to glow at the fullest

brilliance permitted by the voltage present on the lamp bus. The threshold level for perceptible glow may be set by

adjusting syllabic lamp threshold adjust R13. Capacitor C3, once charged, holds its charge between successive audio

cycles, but this charge decays quickly compared to typical time intervals between syllables of speech, so that the lamp

flashes at a syllabic rate.

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