I need a little help with a project. I'm very new to building circuits. I have this NE555 duty-cycle pulse frequency adjustable square wave generator module, which I have connected to a 12V DC source. It is working correctly in astable mode.

I have a counter with a signal line that is sending 9V DC and needs to be pulled low to increment the counter. On each pulse of the module, I would like to increment the counter. However I don’t think I can attach the signal line directly to the 555 output as it is sending 9V; do I need a relay? Or maybe I’m wrong. Any assistance would be greatly appreciated. Thanks in advance!

  • \$\begingroup\$ I suggest rewriting the second paragraph entirely. As written it seems to suggest that the counter sends or outputs 9V and that you want to short that output to ground while it's trying to output 9V - which would destroy the device. Rewrite pls. \$\endgroup\$ – scorpdaddy May 8 '19 at 18:43
  • \$\begingroup\$ It is written correctly; the counter itself outputs 9v DC that I need to short to ground each time the generator pulses. \$\endgroup\$ – Marc R Davis May 8 '19 at 18:56
  • \$\begingroup\$ I think we need to see some documentation for this counter. Please provide a link to the datasheet. \$\endgroup\$ – Elliot Alderson May 8 '19 at 21:14
  • \$\begingroup\$ The counter circuit is part of the mainboard of an IGT S+ Slot Machine. I am attempting to reproduce the behavior of the hopper coin-out optic sensor that registers when a coin has been dispensed. The wiring diagram is shown on page 59 of The IGT Manual. The information I have from a reputable source is the coin-out line is normally high (9V DC) and the optic sensor pulls it low when a coin passes by the optics. \$\endgroup\$ – Marc R Davis May 8 '19 at 21:33
  • \$\begingroup\$ Here's an example of a 555 driving a relay electronics.stackexchange.com/questions/336715/… \$\endgroup\$ – stark May 8 '19 at 21:37

enter image description here

Figure 1. The circuit in question.

The Coin Out line is an input to the machine as it is connected to pin 13 of a 4021 shift register and pin 13 is an input.

I have a counter with a signal line that is sending 9 V DC and needs to be pulled low to increment the counter.

Not quite. The input has a pull-up resistor to hold the input voltage high (+9 V) when the input is disconnected. CMOS chips such as the 4021 should not be left with inputs floating as they are susceptible to stray voltages. Your external circuit needs to pull the input low (connect to ground). This can be done with an NPN switch if the ground of the external circuit is connected to the ground of the machine.


simulate this circuit – Schematic created using CircuitLab

Figure 2. Transistor and relay options. Ground and chassis symbols used to denote the separate grounds.

The relay option has the advantage of complete electrical isolation.

| improve this answer | |
  • \$\begingroup\$ Thanks for the clarification on the behavior of the 4021 @Transistor; I ran a test and pulling that pin low (chassis ground) does increment the coin counter as expected. If I go the NPN route - what transistor do you recommend? \$\endgroup\$ – Marc R Davis May 9 '19 at 0:25
  • \$\begingroup\$ Any small-signal NPN transistor will do. 2N2222 is popular. \$\endgroup\$ – Transistor May 9 '19 at 8:19
  • \$\begingroup\$ Thanks! The relay option worked perfectly. \$\endgroup\$ – Marc R Davis May 12 '19 at 3:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.