This circuit has the intensity of the signals controlled by the light that falls on the sensor. Note that it is not a frequency-controlled oscillator. Equivalent transistors can be used.
In the figure we have the way to obtain a monostable action by lighting an LED for a certain time...
This General Electric circuit is from the 80's. The PUTs used in this meter are not common...
We found this circuit with several types of sensors in a 1995 documentation. The circuit also has...