I had a client bring me a high capacity air compressor for PCP Airguns for repairs. It is a PCP Kong Lil Foot and won't start up. I have done up a schematic (reverse engineered) and there is an IC that is unidentifiable. The chip seemed to be functioning when I removed R4 so I kind of figured the transistor was pooched. I replaced the tranny and it still had the same problem. No 5 volts on the output of the IC to turn the relay on and start the motor. I removed the IC and found a set of numbers under the IC, 9CF2B04N and 0139D. No go on finding that datasheeet to identify it. I thought at 1st it was a logic gate of the XOR flavor. But the truth table for an XOR would have the motor relay still running when the tank is full and the power button OFF.

Some how now the IC is seeing 10.5 V at the VCC pin and also if the sw1 and sw2 pins are open (floating) they also see 10.5 V and so the VREG is pooched now, LM78M05. But I cannot find a logic gate that will fit this bill. An op amp (comparator) doesn't fit the bill as far as I can tell either.

What I have managed to do is figure out that a 3 input OR gate will do the trick with input C permanently grounded. I include an Excel spreadsheet showing this.

Does anyone know what kind of IC this is?

In the schematic I used to show pins 5 and 7 grounded. Not so. After removing the IC from PCB these pins were not grounded. However they are shorted internally to pin 8 which is ground. I'm thinking this should not be. After removing the IC and was able to see clearly that pins 4, 5 and 7 have no traces connected and are NC.


3 input OR gate

  • \$\begingroup\$ Could you add a board picture ... \$\endgroup\$
    – Antonio51
    Jan 17 at 11:42
  • 1
    \$\begingroup\$ Was there no motor snubber or freewheeling diode to protect 78M05 input and relay contacts? Seems "made as cheaply as possible." P.S. Dave Jones has done some videos about $0.1 and $0.03 microcontrollers. \$\endgroup\$
    – rdtsc
    Jan 17 at 12:58

1 Answer 1


As supply are at pin1 (Vcc) and pin8 (Gnd), pin4 not used, ...
I should think for a microchip tiny as PIC12F1822 ... for replacement.
Should be programmed easily through pin 4 (Mclear, Vpp), 6 (icspCLK), 7 (icspDAT).

  • \$\begingroup\$ Interesting approach \$\endgroup\$
    – Journey
    Jan 18 at 1:58
  • \$\begingroup\$ After looking at the datasheet ... I think you nailed it my friend. What do you think of my idea to use 3 input OR gate as per function table in the Excel spreadsheet capture ? \$\endgroup\$
    – Journey
    Jan 18 at 2:02
  • \$\begingroup\$ Gonna build it and test it to see if it will work. If input C is always GND then the highlighted rows will only rule the scene. Oh and Thank You very much for your very helpful input and experience \$\endgroup\$
    – Journey
    Jan 18 at 2:08

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