This circuit was published in an article in Radio Electronics magazine in October 1988. It uses components that are still common today, except for the sensor, which is a special type from that time.
The basis of this circuit is the HA2400 released at the time. The circuit is from a Radio...
This circuit with TTL output was found in a 1968 EEE. The transistor can be of general use. The...
This circuit can be found in a 1970 Motorola documentation. The circuit can be implemented with general...