Indicator for Christmas tree (CB20942E)

We found this circuit in a Popular Electronics from the 80s. The circuit is based on the LM3909, a low voltage component for driving LEDs.

Photoelectric Relay (CB4387E)
This circuit was obtained from a 1962 magazine. It can still be mounted to be powered from 6 V...
Simple level indicator (CB9494E)
This electronic level or plumb indicator has two mercury switches as sensors. The indication is...
Gain 100 Amplifier CA3140 (CB25190E)
This inverting amplifier has a transistor output stage that can increase its current capacity. Higher...