A very simple logic probe tester circuit can be build using this schematic diagram .
The logic probe tester circuit is based on two transistors , a npn and a pnp transistor . When the logic probe circuit detects Low the pnp transistor ( BC557 ) is turned on and the green LED will light and when the logic probe detects High the red LED will light . The circuit can be powered using a 3 volts power supply or a battery . You can use a 3 volts battery because the circuit has no current consumption if the logic probe don’t touch anything .