PLC input circuit

This is a work project (again). Sorry I can't share with you a lot of circuit details but I can describe the global working.
We had an issue with an PLC input circuit. A piece of equipment with a mains power output (230V ~) gives a signal to a PLC in another machine. The two machines are connected by means of a relay. Most of the time this works reasonably well. However, sometimes the PLC sees multiple triggers because of relay bounce.
Five years ago, I developed this circuit it is supposed to function as a electronic signal relay. An issue was that there was no additional power available to power the electronics. The PLC input circuit is standard circuit with a 24V output in series with an unknown resistor. So to test the power available from input of the PLC I checked with several resistors whether they would trigger the input. The result:

Resistance	Trigger (J/N)
470	J
1k	J
4k7	J
10k	N

Conclusion the output of my circuit should look like a 10K resistor (or larger) when not triggered.  This means that the total power consumption should be 24V/10K < 2.4mA....
Unloaded the input of the PLC produces 23.8V, loaded with a 10K resistor the output voltage drops to 23.1V. Using the formula for a voltage divider (V_out = V_in * R₁/ (R₁ + R₂) ), this indicates an output resistance of circa 300 ohms. Also the short circuit current can then be calculated to be around 23.8V /300 ohm is 79mA.


First setup

The circuit is build around the following modules:

1. 230V impulsgenerator and opto-isolator. The purpose of this module is to generate 50Hz pulses as long as 230V mains is applied.
2. Retriggerable Monostale multivribator with a period > 20msec. The input of this block are 50Hz (= 20 msec) pulses. So as long as those pulses are available at the input a steady low pulse is generated at the output. 
3. Differentiator and inverter. This block generates, at the start of the the long output pulse of the MMV, a short positive pulse. The inverter then inverts this, to a short negative pulse.
4. A Non-retriggerable Monostale multivribator with an adjustable period. Triggered by the inverter a pulse is given with a fixed  but adjustable time period. The period is independent of the length of the input pulse and can be configured using a multi turn potmeter.
5. The output stage is switched on by the 2nd MMV en simulates the switching action of the relay by creating a short circuit for the 24V input.
6. The power supply block converts the 24V to a stable and consistent 12V suitable for all components. A Graetz diode bridge is used for easy connection of the input.

The circuit was so successful it ran for over six years without problems. Now the time has come for multiple circuits so a PCB was designed and ordered. Below, you'll see the populated PCB:

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