This circuit does not require a symmetrical source. This allows the supply to be even made with a lower voltage than that shown on the CIR4021. In the figure we have the configuration that also activates an indicator LED. To obtain the reference voltage equal to half the supply voltage, a divider with resistors R2 and R3 is used. The values of these resistors are not critical and as long as they are the same they can typically be in the range of 10k to 47k ohms. In this case too, it is possible to use an exciter step with a transistor to control higher power loads, such as a relay.
