Using circuits from Analog Devices, this circuit does not need a symmetrical power supply. It is also offset adjustable and is based on the LT1498 and LT1167 op amps.
This circuit detects the presence of a pilot flame in heating systems by the presence of infrared...
In the figure we have a circuit that uses unusual components, being indicated for low frequency...
Found in older Signetics documentation, this circuit generates signals from 30 Hz to 25 kHz....