Simple Logic Probe

Now you might think all you need to check logic levels in a digital circuit is an LED. If it lights up you're measuring logic '1' (HIGH) and if it doesn't light up you're measuring logic '0' (LOW). This may be so, but if the probe is open circuit (not connected to anything) the LED still wouldn't light up, so you won't really know the true state of what you're measuring. This circuit indicates 3 different states: HIGH, LOW and OPEN. It operates by using a bistable oscillator formed by two inverters (NOT gates) with RC feedback. The two LEDs are actually flashing back and forth between each other but the component values have been chosen so that the speed is so fast they appear to both be on at the same time. This is the normal state of the circuit with the probe not connected to anything. Effectively the oscillator is giving the gates something to do while the input is 'OPEN'. Now if the input is taken either 'HIGH' or 'LOW', the inverters simply operate as they would normally... a '0' on the input giving a '1' on the output and a '1' on the input giving a '0' on the output which the LEDS will indicate (the diode in the supply rail is for reverse polarity protection)

Green LED on / Red LED off = Logic level 0 or LOW

Red LED on / Green LED off = Logic level 1 or HIGH

Green and Red LEDs on = No connection or OPEN

Note: It's good practice to leave unused gate inputs connected to something