In order to connect the open-collector output of a standard Conrad DCF receiver module (order number 121177), which has a relatively low operating voltage of 1.5–15 V and an NPN open-collector output, to a PLC requiring a positive-switching 24-V signal at around 150 mA, level shifting is necessary. This can be achieved in a simple manner using a miniature relay connected between the positive supply voltage for the module and the DCF module output, as shown in Figure 1. In this case, the current load on the DCF output must not exceed 1 mA. All that is needed for this solution is a 15-V miniature relay with a suitable coil voltage that does not require the DCF output to sink more than 1 mA. There are no special requirements on the PLC side since the contacts of just about any relay can switch 24 VDC at 150 mA. Besides the electromechanical solution, there is also an electronic solution.
Two resistors and a transistor are all you need, as shown in Figure 2 if you are prepared to do without electrical isolation between the two devices. Any desired type of Darlington transistor without a built-in resistor can be used, as well as a p-channel MOSFET. This means that a BC516 or BD680 is suitable, but not a TIP142! The circuit grounds of the two devices are connected together. Naturally, this compact level shifter can be used for other applications as well; the application shown here illustrates how an open-collector output can be connected to a system having a different supply voltage.