This circuit with TTL compatible output was found in a 1968 US documentation. The circuit is powered by 5 V and the frequency can be changed by changing the crystal.
This circuit can be implemented with discrete transistors. We found it in General Electric's 1964...
The operational amplifier used on this Raytheon circuit is similar to the 741. The circuit is from...
This circuit detects light variations using two LDRs as sensors. Operational amplifiers and...