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Old September 27th, 2021, 12:36 AM   #16

lwmar is offline
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Ok, I understand that circuit of post 13 will not work as I will need a transistor. I am still unable to solve this. On the last circuit I posted, when measuring with the virtual instruments of the Proteus simulator I can see a very low voltage at the connection point to the PLC but in real life I measure between about 5 and 12V. As I am using all the other inputs of the PLC connecting them to the negative of the 24V power supply when I want them high and leaving them floating when I want them low, I think it is not good to have such voltages. Parky you mentioned a PNP transistor, do you mind drawing a circuit for me please?

Another way Parky mentioned before: Using a relay between the transistor and the PLC. I have a relay that uses 12V in the coil and another one that uses 5V there. I measured 362 ohms on the coil of the first one and 0.2 ohms on the coil of the second one. If I connect the 12V relay to 5V I can hear it clicking so my circuit is still not good for it. If it is easier for you probably you can draw a circuit that includes one of this 2 relays?

Using Multisim I managed to simulate the following that it works the way I want as when the LDR resistance changes to 35kohms the light bulb turns off. (Please note that in this new circuit I had to move the LDR to the top). I am also including the circuit showing how I plan to connect it to the PLC.

Please give me your thoughts about it. I am still wondering how this can be done using only solid state electronics. Thanks
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File Type: png LDRtoRelayfoPLC01.png (29.9 KB, 6 views)
File Type: png LDRtoRelayfoPLC2.png (24.6 KB, 6 views)

Last edited by lwmar; September 27th, 2021 at 01:22 AM.
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Old September 27th, 2021, 01:34 AM   #17
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drbitboy is offline
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Can you provide documentation for the digital input on the clone?

Is there a manual that provides sample or suggested wiring diagrams?

Can you provide the specifications for the LDR?
i) Take care of the bits, and the bytes will take care of themselves.
ii) There is no software problem that cannot be solved with another layer of indirection.

Last edited by drbitboy; September 27th, 2021 at 01:54 AM.
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Old September 27th, 2021, 03:35 AM   #18
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The resistance of those coils does not seem correct so I suspect that one of them is an AC relay, hence the low coil resistance, a 12v relay driven off 5 volts is not a good idea, the induced magnetic field will be low perhaps that is why it chatters.
Here are some basic circuits, you will need to adapt them with resistor values to suit, if you have a transistor simulator that should be no problem, TBH, it's been over 40 years since I did any real work in discrete transistor circuits & lost quite a few brain cells since then.
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File Type: png simple-pnp-transistor-circuit.png (11.9 KB, 6 views)
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Old September 27th, 2021, 11:47 AM   #19

lwmar is offline
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Thanks for the answers.
I am using the Mitsubishi FX2N manuals I've found on the web as this clon does not come with any pdf or printed manual or leafleat or URL or anything at all.
The LDRs are generic and do not have any marks, so I measured them with a multimeter and the one I am using now shows this values:

32 kohms when the laser is not illuminating it
3.2 kohms when the laser is illuminating it
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