Found in a1967 Ferranti documentation, this circuit uses an operational amplifier from that time. The circuit uses a LDR as a sensor and has a null setting.
This circuit was obtained from an American 80's documentation. The balanced mixer can be shown in...
This circuit produces a square wave signal with the frequency determinated by R1 and C1. The upper limit...
In the figure we have the circuit of an amplifier for alternating signals with 10 times gain,...