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Random Access Footswitch - Printable Version

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Random Access Footswitch - Strelok - 10-13-2021

Hi guys,

In TUT2 there is a random access footswitch, built around a 74175.

It works perfectly with alternate switches set up as momentarily.

Very cleverly done!

I'm trying to use normally open momentaries but I can't get it to work.
I tried moving the 10K pull-ups to become pulldowns by connecting them to the ground rail, but no go.
Can this only be done with normally closed momentaries?

Or do the diodes need to change direction?

Talk to you later,


RE: Random Access Footswitch - K O'Connor - 10-14-2021

Hi Strelok

With the less expensive normally-open/closed push buttons a debounce circuit is required. Our QRK-LATCH kit incorporates this, using three BJTs per switch. You can also use CMOS inverters for the task, or try to go passive. In all cases, a small capacitor is required to provide the controlled rise without noise.

One form of the passive circuit capacitively couples the switch to the chip input. On the chip side of the cap is a resistor to ground to assure a LO input as standard. The switch closes and ties the cap to V+, making a pulse into the gate of the chip.

An alternative form uses a NC switch in parallel with the cap to ground. A resistor to V+ charges the cap when the switch opens.

In both passive cases the CMOS threshold voltages for recognition of a HI or a LO determine the point at which the chip changes its status.

The active circuits provide a narrow square pulse hits the latch chip.

Have fun

RE: Random Access Footswitch - Strelok - 10-15-2021

Hi Kevin!

Thanks for your elaborate answer!

Did not know there was so much about it haha.
Debouncing, I considered it but I thought once HI was hit one time the state would remain the same.
Only when another switch was hit would the state change, in my idea.

I will check your QRK-Latch kit.

Alternate action switches make a lot of mechanical noise, that is why I want to avoid them.

Kind regards,


RE: Random Access Footswitch - K O'Connor - 10-19-2021

Hi Strelok

"Alternate action switches make a lot of mechanical noise, that is why I want to avoid them"

They should NEVER be used to switch audio signals directly, as the "true bypass" crowd insists on doing because they do not understand how to build a proper switching circuit. Wired as a pulse switch per TUT, the mechanical noise of the switch is not an issue although it is a rather expensive way to do things.

As I said, debouncing does not have to be complicated.

Have fun