Matching a sensor interface with a programmable logic converter or controller hardware that can accept only one type of input—npn or pnp—can be difficult. The last-stage modification circuit described here solves this problem. Designers can use it to convert an npn input to a pnp type or incorporate it in the last stage of the sensor design to generate both npn and pnp outputs.
The npn and pnp transistors used in the circuit should be high current- gain (hfe > 100) devices with minimum 2-A continuous current drive capacity (see the figure). This ensures an output stage that can directly drive a relay with a 22-V switching voltage from a 24-V power supply. The transistors used (a 2N3055 npn and 2N2955 pnp in TO-220 packages) are reasonably good choices. They should be placed on a heat sink for good heat dissipation.
The npn first stage operates from a TTL-level input: a 0.4- to 2.4-V signal with a 20-mA maximum current from the signal. To translate the npn signal into a pnp signal output, the first stage drives a pnp switch to create a high-current source of about 22 V.
The positive temperature-coefficient (PTC) thermistor in the pnp transistor’s current-control circuit prevents short-circuit damage. A short circuit would create excessive current and the resulting heat will cause the thermistor's resistance to rise sharply. Once cooled, the circuit starts working again, so the thermistor acts like a resettable fuse.