window' LEDs - easy-to-make reference circuit

easy-to-make reference circuit It is often interesting (if not required) to know whether an amplifier is going into saturation, or whether certain limiting values (thermometer, power supply, etc.) are being ex- ceeded. It is, however, not always feasible to use a fully-fledged win- dow discriminator and in those cases the circuit presented may be of interest. When the level of the input voltage lies between 3.5 V and 8.5 V, tran- sistors T1 and T2 conduct (T3 and T4 are cut off) so that LED D1 lights to indicate that the input signal is "in range". When the input level rises above about 8.5 V, T2 and T3 conduct (T1 and T4 are cut off) which causes LED D2 to light indicating that the input level is "above range". Finally, when the input signal drops below about 3.5 V, T1 and T4 con- duct so that LED D3 lights indicating that the input is "below range. The current consumption is for all practical purposes governed by the LED currents which are 20 mA maxi- mum: it may briefly rise ab...