Darkness Monitor (CB21019E)

Found in Popular Electronics this circuit triggers an SCR and lights an LED when the location of the sensor is dark.

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Car Lights On Reminder (CB4786E)
This circuit was found in a 90s Hands-on Electronics. The transistor can be a BC548. What it does...
Electronic Crossover (CB20648E)
This circuit separates the bass from the treble in an audio signal. The circuit is from a Hands On...
Temperature to Frequency Converter (CB12218E)
The frequency of the signal produced by this circuit depends on the temperature of the diode used...