[SOLVED] Parallel switches (one on PCB, another on the bench/panel)

Doesn't make much sense. May be the switch is placed 180° rotated so that the switch is normally closed. Suggest to use a multimeter to check the switch states and voltage levels. "output signal does not change" is unspecific, what's the voltage level at the RC filter? 100R pull-up is unusually low, expect 2k to 10k.

FvM said:

May be the switch is placed 180° rotated so that the switch is normally closed. Suggest to use a multimeter to check the switch

Click to expand...

The external switch has only two pins, so it can not be connected in a wrong way. But the linked external switch is a normally closed one. Which kind of logic IC are you using, a D-FF? On which edge is it triggering, rising or falling? Has it a schmitt-tigger input?

BR

FvM said:

Doesn't make much sense. May be the switch is placed 180° rotated so that the switch is normally closed. Suggest to use a multimeter to check the switch states and voltage levels. "output signal does not change" is unspecific, what's the voltage level at the RC filter? 100R pull-up is unusually low, expect 2k to 10k.

Click to expand...

Only when I press and hold S1, I can see a voltage (4.94 Vdc) relative to GND at pins 0 and 2 (pin 2 is not used) of the S1. Votlage accross RC filter is 4.88 Vdc.

What confuses me is that when S1 is not pressed, there is no voltage at any of the switch S1 pins relative to GND. I am not sure why is that and how to achieve that with another external parallel switch.

When referring to the "output signal", I mean the output of the D-Type Positive Edge-Triggered Flip-Flop 74ACT74.

stenzer said:

The external switch has only two pins, so it can not be connected in a wrong way. But the linked external switch is a normally closed one. Which kind of logic IC are you using, a D-FF? On which edge is it triggering, rising or falling? Has it a schmitt-tigger input?

BR

Click to expand...

This is the main part of the schematics. D-Type Positive Edge-Triggered Flip-Flop 74ACT74 output 6 is used as a control signal (START, RESET, STOP).
When S1 is pressed, there is a voltage across S1 pins 0 and 1 (if referenced to the switch picture; on schematics, those are pins 1 and 2) and this changes the output state (74ACT74's pin 6).


For S1 it says (ON)-(ON) momentary switch, and now I am trying to add another switch in parallel to use it on the bench or panel.
I have another external momentary switch. When pressed, the output resistance is 0 Ohm, and when released multimeter shows OL. When connected to J3, it does not change the output signal (74ACT74's pin 6)

mike buba said:

Only when I press and hold S1, I can see a voltage (4.94 Vdc) relative to GND at pins 0 and 2 (pin 2 is not used) of the S1. Votlage accross RC filter is 4.88 Vdc.

Click to expand...

There's no pin 0 in your schematic. Nevertheless, this sounds like NC operation of the switch, opening on key-press, switch reverted as guessed above.

I reviewed post #1 and found that the switch is connected incorrectly. 0 is actually COM, 1 NC and 2 NO. Respectively 0 and 2 should be connected to the input. Rotating the switch by 180° may do the trick.

FvM said:

Rotating the switch by 180° may do the trick.

Click to expand...

So the schematic works as it is, i.e. I get what I need: every time S1 is pressed, flip-flop output changes state.

When I try to add an external momentary switch with two pins on J3, the externl switch doesn't work, i.e. flip-flop output doesn't change the state.
Should I be using the same three-pin switch and connect it like S1?
---
Quick update after some more tests.

When S1 is closed (I am holding down the pushbutton), then another pushbutton connected to J3 works, i.e. I see the change in the flip-flop output when pressing J3.

But then, I cannot hold S1 closed all the time.
... all this still doesn't explain why there is no voltage at any of the S1 pins when the pushbutton is not pressed, and when pressed there is ~5Vdc between two connected pins.

mike buba said:

When S1 is closed (I am holding down the pushbutton), then another pushbutton connected to J3 works, i.e. I see the change in the flip-flop output when pressing J3.

Click to expand...

Hi,

This is what others already guessed: --> the swich is not CLOSED when pressed, but OPEN when pressed.

Klaus