We have 2 potential chargers:

The chargers are connected to charging pads, which are basically bare copper pads with a spring and limit switch (NO) internally. A robot comes in and docks on the pads closing the limit switch to commence charging.

The PB-1000-48 only starts charging when it detects a voltage source, the NPP-1700-48 works more like a power supply, once it's turned on it just puts the charge voltage on the output.

Additionally, they both turn on in exactly opposite ways. The NPP charger requires shorting 2 pins on its connector, or providing 10.8-13.2V on pin 7 to work:

enter image description here

Whilst the PB charger expects two pins to be open circuit for it to work:

enter image description here

Expected Operation

The way the circuit will work is that once the robot docks and the limit switch(https://docs.rs-online.com/1fc1/0900766b80dd6a18.pdf) (NO) will be pressed and once the robots battery voltage is detected (40-56v), an appropriate signal would be sent to the charger to turn it on.

The battery voltage detection is only required for the NPP charger since the PB manages that on its own.

Finally in the case of the NPP charger, once the two conditions are met (battery voltage and limit switch) a delay (0-5s) adjustable through a pot would be added before charger begins charging. There should be minimal delay at turn off.

I have simulated something in LT spice which does the job but i was hoping if some improvements, suggestions, comments could be added. I am not sure if this has been over complicated.

The SPDT shown here (u4) is simply a 12V relay. So i can use this for the PB charger. I couldn't use a 48V relay directly on the output as it needs to come after the limit switch and the limit switch is rated to 30V. The mosfet circuit shown is just a buffer circuit (CD4050UB) and i just used the logic block here as shown: enter image description here

The buffer was added mainly for the RC delay circuit with R being a 500K POT (R45) here and C (C5) being 10u. enter image description here


  • Both chargers will never be connected across at the same time.
  • Once the robot is off the charger, and limit switch released, the charging would stop.

Any help to refine/improve/simplify this circuit and any obvious issues would be greatly appreciated.

  • \$\begingroup\$ Don't leave M20, M21, M24, M25 as default NMOS and PMOS. The default models use Vto=0. Pick something from the built-in list like you did with the others, or go find the actual CD4050 model. \$\endgroup\$
    – Ste Kulov
    Jan 3, 2023 at 18:08
  • \$\begingroup\$ Thanks for pointing that out. I missed it. \$\endgroup\$
    – Hasman404
    Jan 3, 2023 at 19:34

1 Answer 1


A single component can do most of it. Just use a relay with a normally closed contact to convert from a normally open condition to a normally closed condition.

For the delay, you may want to just buy an off-the-shelf timer module (cheap, readily available from Amazon and such, easy to configure, guaranteed to work out of the box), rather than spinning your own circuit.

  • \$\begingroup\$ I do need the delay after the limit switch though. So it wouldn't exactly be a single component. Also the limit switch itself is rated upto 30V and i can't change it. Could you elaborate on your solution? \$\endgroup\$
    – Hasman404
    Jan 3, 2023 at 17:04
  • \$\begingroup\$ When the external battery source is removed the charger would still detect a voltage (the voltage from the charger), this is where the limit switch comes handy too to basically cut the circuit. \$\endgroup\$
    – Hasman404
    Jan 3, 2023 at 17:07
  • \$\begingroup\$ " the charger would still detect ... the voltage from the charger". Sorry, I don't understand. Why would the charger detect its own voltage? \$\endgroup\$ Jan 3, 2023 at 17:09
  • \$\begingroup\$ As in the charger is on the same line as the battery, once the charger turns on, that voltage would still be there. We would need to look at the limit switch to know that the battery is no longer present and we should cut power to the charger. \$\endgroup\$
    – Hasman404
    Jan 3, 2023 at 17:11
  • \$\begingroup\$ And i suspect the off the shelf timer module would be more expensive. \$\endgroup\$
    – Hasman404
    Jan 3, 2023 at 17:12

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