relay jack to looper?

[eleanor296]

Hello!

I'm in the possession of a Hughes and Kettner Access preamp which has two switch jacks on it.  They basically work like standard footswitches.  My question is, how can I utlize that to trigger a relay?
I want to use the relay to switch a loop into the signal path, as seen here:


Thank you guys so much!

Andy

Edit:
I think I found what I'm looking for, of course from RG:


Now my question is just... how do I use a footswitch to drive that?  I don't understand what's mean with the +On/-Off signal...


Edit:
Here's a circuit snippet posted by Aleksi:

Would it be ok to substitute a 2n3904 for the BS170?  I got some of those lying around.

[composition4]

Yep but you'd want to get rid of R2 as you don't need a pulldown for a BJT like 2n3904

and you'd need to put in a suitable base resistor just before the base of the transistor to stop it burning up

Let me know if you need help with the calculations (Search "calculating base resistor" or something similar on the forums for a start)

Jonathan

[eleanor296]

Yep but you'd want to get rid of R2 as you don't need a pulldown for a BJT like 2n3904

and you'd need to put in a suitable base resistor just before the base of the transistor to stop it burning up

Let me know if you need help with the calculations (Search "calculating base resistor" or something similar on the forums for a start)

Jonathan

Thanks a lot man!

So far I've mostly been just tinkering with stuff like this, though I'll start my circuit classes next semester.

Edit:

So I'd use R_b=(V_c x hfe)/(5x Ic)=(9V*hfe)/(5*200mA)... but what would I plug in for the DC Ccurrent gain?  The maximum value of 300?

[PRR]

> what would I plug in for the DC Ccurrent gain?  The maximum value of 300?

Then only the HIGHEST-spec transistor would work.

Generally when the transistor is used as a switch, you assume hFE near "10", so that anything from 50 to infinity will for-sure work solid.

This is all you -need- :



The question is: can the footswitch stand repeated 12V/47= 0.25A jolts?

[Brymus]

Couldnt you just leave the 4k7 in there ? (for .002553 amps)
And doesnt it just need a connection to 12V to turn it on and breaking that connection to turn it off ?
Sorry if those are dumb questions ,I havent really looked at using transistors as switches.

[eleanor296]

> what would I plug in for the DC Ccurrent gain?  The maximum value of 300?

Then only the HIGHEST-spec transistor would work.

Generally when the transistor is used as a switch, you assume hFE near "10", so that anything from 50 to infinity will for-sure work solid.

This is all you -need- :



The question is: can the footswitch stand repeated 12V/47= 0.25A jolts?

ah, thanks for the clarification there!
Hmmmm well it's a tube rack preamp that's pretty rare, so I'd rather not risk damaging it in any way.

[eleanor296]

Well guess I'll stick with Aleksi's schematic.
Also not a problem :-)

In his thread he said that R1, the 1K resistor, should be omitted, but he never mentioned why?

Edit: NVM, I understand now.

It was suggested that he add a Zener diode from gate to ground, but exactly what would the value of that Zener be/how can I find it?

[composition4]

The Zener is to protect the MOSFET (BS170) from over-voltage on its gate.  If the voltage goes over the Zener value, it will be shunted to ground as the zener will conduct at that breakdown voltage.

So you want a value that will protect the MOSFET's gate, as it can only handle up to around 20v (I choose a lower value to be safe), but the Zener needs to be high enough that it won't conduct in normal operation voltages.

If your V+ is 9v, I would choose between 12 - 15v or somewhere thereabouts

Jonathan

[eleanor296]

The Zener is to protect the MOSFET (BS170) from over-voltage on its gate.  If the voltage goes over the Zener value, it will be shunted to ground as the zener will conduct at that breakdown voltage.

So you want a value that will protect the MOSFET's gate, as it can only handle up to around 20v (I choose a lower value to be safe), but the Zener needs to be high enough that it won't conduct in normal operation voltages.

If your V+ is 9v, I would choose between 12 - 15v or somewhere thereabouts

Jonathan

Thanks for the great explanation.  It's great that I can learn so much here on this board.

[composition4]

oh just one other thing, the Zener needs to be REVERSE biased (cathode/band AWAY from ground, anode towards ground) as it's the reverse breakdown voltage you'll be using it for

Jonathan

[Brymus]

I would like to see the finished schematic of what you use,once its verified.
This would work for other switching needs too.

[eleanor296]

Hey Brymus, here's my modified version of Aleksi's schematic.

composition4 or someone else who knows what they're talking about, is that right?

[eleanor296]

aww, nobody?  It's based off of Aleksi's schematic, I hope he doesn't have a problem with me using it...

[composition4]

Yep that looks good to me

I like to use back to back Zeners for mosfet protection but I'm sure it will be fine that way anyway

Just make sure you're using a 9V relay - if you are using a lower voltage relay you should figure a suitable resistor in series with the coil

Jonathan

[eleanor296]

Yep that looks good to me

I like to use back to back Zeners for mosfet protection but I'm sure it will be fine that way anyway

Just make sure you're using a 9V relay - if you are using a lower voltage relay you should figure a suitable resistor in series with the coil

Jonathan

Alright man, thanks!

By back to back you just mean two in series?  What's the reasoning behind that?

[composition4]

My understanding is that it will more safely handle positive and negative discharges in relation to gate-source voltage, whereas single zener would be more about overvoltage protection primarily.  Probably fine with a single, but for small-scale stuff I don't think an extra zener used here and there is going to break the bank.  Someone on this forum works with ESD as their main job (can't remember who) and could tell you more about it or if I'm correct

Jonathan