The purpose of this circuit is to convert a short-lived pulse into a negative rectangular pulse. The circuit uses a unijunction that can be replaced by an equivalent such as 2N2646.
The recommended distance between the emitter and the phototransistor is 10 mm for this...
This circuit was found in a 1994 documentation. Speed measurement pulses are obtained by a reed...
Here is a high-quality intercom for demanding readers. The circuit is from a 1982 documentation. Of course,...