A Switching Scheme

mictester · Post by **mictester** » 05 Jun 2011, 19:37

Groovenut wrote:
dukie wrote:
Groovenut wrote:on-latching version of this relay. That is your issue. I have successfully tested this circuit with AL5W-K and 2N3904/2N3906 and it only draws ~60uA current when on. Try switching to the latching version. That should cure your problem.
Btw, If I'm using the latching relay then should I use momentary switch or just ordinary SPST switch?

Cheers!
With the posted circuit you need to use a latching switch. If you want to use a momentary switch, you have to use an electronic latch of some sort.

Exactly right. I have built them using 4013 or 4027 (or even 4093) flip-flops to replace the series switch. The CMOS draws minute current itself, and you know how little current is pulled by the relay circuit... The effect circuit pulls almost all of the current drawn from the battery!

mictester · Post by **mictester** » 05 Jun 2011, 23:39

OK.

A quick play with DIYLC came up with this layout. I built two like this tonight, so it works fine! The boards were fixed to the back of the footswitches.

: Quite small!; Low_Current_Relay.gif (14.04 KiB) Viewed 3873 times

There were mounting holes either side of the relay, so the board was left a bit bigger than it needed to be.

soggybag · Post by **soggybag** » 08 Jun 2011, 22:37

mictester wrote:You'll all know my hatred of the crappy TPDT and similar footswitches.

I still have customers who insist on "true bypass" and a "solid copper path bypass". Buffered bypass is almost always better, but just to satisfy these (very wealthy / influential) musicians, I frequently use bistable relays. Many players are deeply distrustful of mains powered effects, and (perversely) demand batteries that last for many hours. I also have a large supply of really sturdy SPDT footswitches (like the old "Carling" ones). To overcome the deficiencies of the switches (they crackle and can't do proper bypass), I've been using the following circuit. The relay is easily available from all the major suppliers (including many Ebay ones) and if used in this circuit, draws micro-amps!

Ultra-low_current_relay.png
The relay is slightly unusual - it has two stable positions, and is set and reset by small current pulses sent in one direction then the other through the coil. The transistors I used in the originals were BC183L for the NPN and BC213L for the PNP. You can also use 2SC1815 and 2SA733 or BC550 and BC560. The last batch of them had blue LEDs wired across the anode of the diode to ground through a 8k2 resistor, and used 2N3703 (PNP) and 2N3706 (NPN), and were made to fit on to the back of the footswitch.

The circuit gives six big advantages:

1. Hermetically sealed gold contacts - clean and quiet switching.
2. Additional relays can be put in parallel with the first one. You might have to increase the 100µF to 220 µF, but the switching will be entirely reliable.
3. Draws minute current - the whole of the battery power will be used to power your effect and the indicator LED!
4. Passes the "disconnected battery" "True Bypass" test.
5. Costs less than a good quality DPDT footswitch.
6. Allows simple connection of an indicator LED.

Great idea, I'm curious about the type of foot switch you are using with this and what is the part number for the relay?

Post by **Brink** » 09 Jul 2011, 06:11

mictester wrote:The only addition I sometimes make to this circuit is to use a CMOS bistable to feed the anode of the diode - the "+" point. I use a 4013 configured with one side as a bistable (pin 1 or 13 feeds the switching circuit directly), and the other side as a power-on reset and debounce circuit, or I use a 4093 quad schmitt NAND with two gates for the bistable, one for debounce and one for switch-on reset. Either circuit then allows the use of a momentary switch.

Do you have a schematic or two floating around?

DeanM · Post by **DeanM** » 21 Jul 2011, 15:56

Recently i was looking at PIC microcontroller driven relay circuits. But with the low current and the fewer parts count that this circuit has its probably better than using a PIC and a momentary switch to do the switching? or is there any advantages to PIC/Momentary?

Also I found this circuit online too.
http://circuitschematicelectronics.blog ... relay.html

LaceSensor · Post by **LaceSensor** » 21 Jul 2011, 16:04

mictester wrote:OK.

A quick play with DIYLC came up with this layout. I built two like this tonight, so it works fine! The boards were fixed to the back of the footswitches.
Low_Current_Relay.gif
There were mounting holes either side of the relay, so the board was left a bit bigger than it needed to be.

Dumb question where would you wire the LED for the status?

Thanks

YuGi · Post by **YuGi** » 21 Jul 2011, 16:20

LaceSensor wrote: Dumb question where would you wire the LED for the status?

mictester wrote: The last batch of them had blue LEDs wired across the anode of the diode to ground through a 8k2 resistor

DeanM · Post by **DeanM** » 21 Jul 2011, 16:20

LaceSensor wrote:Dumb question where would you wire the LED for the status?
Thanks

From diode anode to ground through a resistor to limit current.

See mictesters previous comment>

mictester wrote:The last batch of them had blue LEDs wired across the anode of the diode to ground through a 8k2 resistor

mictester · Post by **mictester** » 21 Jul 2011, 20:03

soggybag wrote:Great idea, I'm curious about the type of foot switch you are using with this and what is the part number for the relay?

The relay part number is TAKAMISAWA AL5WN-K.

The switch I use is (usually) an old "Carling" SPDT type - just like the ones in original Cry Baby Pedals. I have a large number of them that were removed a long time ago when converting Cry Babys to "true bypass". They've been in the "odd switches" drawer for years, and I thought I should use them up!

If you want to use a momentary switch, you have to use a logic bistable circuit. I frequently use a 4093 (I've got loads of them), which is a quad, Schmitt NAND gate. I use two of the gates as a bistable, another one for de-bounce, and the last for power-on reset. If you really want, I'll put the circuit up here with a suggested Vero layout.

The momentary switch I use is a stainless steel "vandal proof", domed button type that's available from CPC and Farnell.

decora · Post by **decora** » 22 Jul 2011, 00:41

i would really like to try this out.
is there an alternative relay? can't find it on farnell's site.

Post by **Brink** » 22 Jul 2011, 04:54

mictester wrote:If you want to use a momentary switch, you have to use a logic bistable circuit. I frequently use a 4093 (I've got loads of them), which is a quad, Schmitt NAND gate. I use two of the gates as a bistable, another one for de-bounce, and the last for power-on reset. If you really want, I'll put the circuit up here with a suggested Vero layout.

Yes, please.

DrNomis · Post by **DrNomis** » 22 Jul 2011, 10:37

I was just doing some rumaging around in one of my bags of electronic bits and pieces, and came across a little relay, here's a pic of what it has written on it:

mictester · Post by **mictester** » 22 Jul 2011, 12:36

decora wrote:i would really like to try this out.
is there an alternative relay? can't find it on farnell's site.

I get the relays from a Panasonic component dealer (I buy them in 50s). You'll see them advertised on Ebay for about $20 for 10 of them or $11 for 5.

Remember - they MUST be the single coil bistable type.

decora · Post by **decora** » 23 Jul 2011, 09:31

Right, thanks. Farnell returns HF3 93S by axicom, doubt i'll get it because the price is just too silly for me. have to wait for the next trip to the big store then. cheers!

DeanM · Post by **DeanM** » 24 Jul 2011, 16:57

Brink wrote:
mictester wrote:If you want to use a momentary switch, you have to use a logic bistable circuit. I frequently use a 4093 (I've got loads of them), which is a quad, Schmitt NAND gate. I use two of the gates as a bistable, another one for de-bounce, and the last for power-on reset. If you really want, I'll put the circuit up here with a suggested Vero layout.
Yes, please.

Another yes please! That would be great. The marshall vintage modern uses 4093 in its dynamic range switching and it also uses d type flip flops. So that would be very interesting!

earthtonesaudio · Post by **earthtonesaudio** » 25 Jul 2011, 19:08

Here's a debounced bistable flip flop that always powers up with its output low. Uses one Schmitt trigger inverter, which can be one gate from a 4093 (shown), or one gate from a 40106, or a single 555 wired as an inverter.

mictester · Post by **mictester** » 26 Jul 2011, 20:58

Here's the bistable I use. The gates can be any CMOS Schmitt inverter (40106, 4584, 4093 with the inputs of each gate connected together). The switch is a momentary push-to-make type. The 6k8 resistor is about right for the LEDs I use, but you might have to increase or reduce for other types. Try to use a high efficiency (ie: low current) LED type, so that you can make that resistor as big as possible and reduce your current consumption as far as you can and still see the LED!

: Copyright Freestompboxes.org 2011; Bistable Switch.png (1.47 KiB) Viewed 3576 times

coldcraft · Post by **coldcraft** » 26 Jul 2011, 21:21

nice copyright there.

YuGi · Post by **YuGi** » 27 Jul 2011, 15:57

mictester wrote:Here's the bistable I use. The gates can be any CMOS Schmitt inverter (40106, 4584, 4093 with the inputs of each gate connected together). The switch is a momentary push-to-make type. The 6k8 resistor is about right for the LEDs I use, but you might have to increase or reduce for other types. Try to use a high efficiency (ie: low current) LED type, so that you can make that resistor as big as possible and reduce your current consumption as far as you can and still see the LED!
Bistable Switch.png

Can you explain how it work? Sorry I don't know yet this kind of electronics.

mictester · Post by **mictester** » 27 Jul 2011, 22:36

Each of the gates is an inverter. These are also called NOT gates - when the input is at logic 1, the output is at logic 0, and when the input is logic 0 the output is logic 1. We're connecting the IC across a 9V battery, so logic 0 is 0V and logic 1 is +9V. There's also one extra, important property about these devices - they are "Schmitt" gates. Schmitt-input gates have the advantage that they have accurately defined switching levels, and an amount of "hysteresis". I really suggest you read the "Art of Electronics" by Horowitz and Hill, because it gives some excellent explanations of the basic stuff.

Anyway, back to the circuit - if the first gate has the capacitor connected between its input and ground in a discharged state, the output of that gate will be at logic 1, causing the 220n capacitor to be charged through the 1M resistor. The second gate will be seeing a high at its input, so its output will be low, holding the 100n capacitor discharged through the 100k resistor. This condition will be stable, the LED will be on, and the final gate will have a logic 1 at its output.

Now, push and release the button....

The 220n capacitor's logic 1 was put on to the input of the first gate. The 100n capacitor charged up, the output of the first gate went to logic 0 which discharged the 220n through the 1M resistor, the output of the second gate goes high, the LED extinguishes, and the 100k resistor puts that logic 1 on to the input of the first inverter and keeps the 100n capacitor charged. The output of the third inverter is logic 0, and this condition will remain stable, until...

You push and release the button again. The charge on the 100n capacitor is drained away into the 220n and the input of the first inverter falls, returning the circuit to the first state...

The "Schmitt" action of the gates guarantees clean switching, and the relative sizes of the capacitors and resistors are just right for reliable operation.

A Switching Scheme [documentation]