[amb]

Any interest in a circuit that lets you use momentary switches (such as the attractive Bulgin vandal-resistant pushbuttons) as the main power switch of an amp or other device? Here is a solution that also has a couple of extra features thrown in.

HeadWize: DIY Workshop > ε24: Power switch driver circuit

 

[Nebby]

I already picked up a latching button, but I would definitely be interested in a few boards since not having to run AC up to the front is a very nice option

 

[error401]

This is the kind of thing that's terribly useful, but simple enough to easily build on stripboard. I like the 'bonus features' though, they might make it worthwhile for me to buy a couple if I ever get around to casing my amps nicely.

A couple comments. The Omron relays are pretty expensive. I think the Panasonic TQ2 series is a direct substitution, and seems to cost about half as much. No reason not to use the cheaper part here, IMO. Also, is a $2.50 18A MOSFET really required to drive the relay? I'm thinking something like the $0.25 2N7000 (200mA) or $0.35 BS170 (500mA) should be sufficient. If a tiny little 2VA transformer is used, it simply can't provide enough current to warrant the IRLZ24N as far as I can see, and besides, the relay coil current shouldn't be more than 100mA. Maybe you have another reason for using this particular part? Spending 1/4 of the total BOM cost on an grossly overengineered FET seems a bit much to me.

Edit: It'd also be nice to have a pads for +12V/GND. Could be useful for other 'standby' electronics, and I think there's room.

 

[grawk]

Would this work for other switch applications, like turning on and off speakers?

 

[amb]

Quote:

Originally Posted by error401 /img/forum/go_quote.gif The Omron relays are pretty expensive. I think the Panasonic TQ2 series is a direct substitution, and seems to cost about half as much.

Sure, you could use the Panasonic part (TQ2-12V). The Omron is available at both Mouser and Digikey (and seems to be easier to find elsewhere as well). Digikey has the Panasonic but not Mouser.

Quote:

Also, is a $2.50 18A MOSFET really required to drive the relay? I'm thinking something like the $0.25 2N7000 (200mA) or $0.35 BS170 (500mA) should be sufficient.

I chose IRLZ24N because I have a boat load of them. These are the same devices used on the σ11 PSU. In the grand scheme of things they are not that expensive, and there's only one in the circuit. You can use an alternative MOSFET if you want, as long as the pin-out is compatible (the 2N7000 and BS170 are not). But IRF510 is fine, for example.

Quote:

Edit: It'd also be nice to have a pads for +12V/GND. Could be useful for other 'standby' electronics, and I think there's room.

I spec'ed a very small transformer, just enough to do the job. The small radiated field of a small transformer is also desirable, but that means there isn't any "juice" leftover to power other circuits.

EDIT: The regulated +12V is available at the T1 connector, so it's already there. there isn't room for another Molex header position, but I'll add a wirepad that connects to the ground plane. So, if you use a larger transformer then you could do what you want.

Quote:

Originally Posted by grawk Would this work for other switch applications, like turning on and off speakers?

Sure. Instead of a "always-on" power transformer, in this case you can have the circuit run off switched power.

 

[thrice]

Looks like fun...board ETA?

 

[amb]

LOL Thrice, you're as impatient as I am. It sounds like there is enough interest for a small run of boards. The layout has passed DRC and ERC, and I've also manually verified all nets against the schematic. So, I do like a fast track to get some boards made. My β24 build needs it...

 

[n_maher]

Quote:

Originally Posted by amb /img/forum/go_quote.gif LOL Thrice, you're as impatient as I am. It sounds like there is enough interest for a small run of boards. The layout has passed DRC and ERC, and I've also manually verified all nets against the schematic. So, I do like a fast track to get some boards made. My β24 build needs it...

Ti, I'm sure you know all you have to do is ask and some of us will be more than willing to help offset the cost of a small production run of these but I want to reiterate it anyway.

 

[amb]

No worries, Nate. I think there is enough interest to get at least 20 boards made. That should make the per-board down to somewhat affordable levels. I think those of us who care about the aesthetics of our build enough to use these Bulgin switches (and shudder at the thought of a perfboard or protoboard inside our amp) wouldn't mind spending a few extra bucks for the vanity.

 

[kipman725]

I would have used a PIC so cost is reduced the the following features would be pretyt easy to impliment:
Remote controll
Power on mute
Dc ofset detection
Output overcurrent detection

 

[MoodySteve]

Consider me interested!

Now all I have to find is a switch that matches the appearance of those Bulgin switches but with gold or brass plating (hooray for vanity!).

 

[amb]

I went ahead and ordered 20 boards. I believe there is enough interest that I wouldn't be sitting on them forever. ETA is around April 9.

 

[fierce_freak]

I've only seen pictures of those anti-vandal switches with the green or blue rings, but I'm interested in using one with a red ring. Does anyone have any photos of that?

 

[thrice]

Allied Electronics sells them and they have a full color catalog, you might check there, but I've only seen the blue and the green in pictures. Those pics amb posted are from the datasheet.

 

[n_maher]

and why not a few others...

There are a fair number of possibilities for cool switches once you open the momentary door.

[edit]Oh dear, thrice is going to need a napkin...