A Switching Scheme

earthtonesaudio · Post by **earthtonesaudio** » 27 Jul 2011, 22:42

{EDIT}Mictester and I were typing at the same time.

{/EDIT}

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

On power up, the 100n cap is discharged (cap voltage=0), so the first inverter output goes high, causing the 220n to charge up to +9V slowly through the 1M resistor. The high level on the first inverter output also causes the second inverter output to go low, which reinforces the low at the input of the first inverter due to the positive feedback provided by the 100k resistor. The LED is on and the "Output" is high.

When you press the switch, the 9V of charge on the 220n cap is dumped onto the 100n cap, causing the voltage at the first inverter's input to go higher than 6V, which is enough to make it's output change state. This turns the LED off and the "Output" terminal goes low.

If you held the switch closed, nothing would happen because the charge on the 220n is only higher than the threshold for a fraction of a second. It quickly is dumped into the 100n and through the 100k resistor. This is also what prevents "switch bounce" to cause false triggering.

YuGi · Post by **YuGi** » 28 Jul 2011, 09:12

Thank mictester and earthtonesaudio, I had confirmation of what I thought (at the power on the state is ON).
Is for that reason mictester that you need the last inverter of a 4093 for power-on reset?
What is the difference from earthtonesaudio schema (only one Schmitt inverter) e this one with three inverters? At me it seems instable if you hold the switch pressed..

mictester · Post by **mictester** » 28 Jul 2011, 19:15

If you hold the switch, it changes state and stays there - the capacitors can't change charge!

It's a reliable little circuit, and variations of it are found in lots of commercial pedals.

earthtonesaudio · Post by **earthtonesaudio** » 28 Jul 2011, 20:01

YuGi wrote: What is the difference from earthtonesaudio schema (only one Schmitt inverter) e this one with three inverters? At me it seems instable if you hold the switch pressed..

Both circuits remain stable as Mictester described. The main difference is that mine has to use a Schmitt trigger and his can use regular inverters.

YuGi · Post by **YuGi** » 29 Jul 2011, 10:09

Example with 9v power supply, as I understand the inverter input is 1 (4.5 V) then the output is 0 (0v): the LED is off and the capacitor is discharged.
When I press the switch the capacitor charges because is connected to 4,5v and the voltage at the inverter input go down for a moment inverting the output status (9v). At this moment the LED is ON but if I hold pressed the switch the capacitor charges until 9v, the inverter input "sees" 9v and the output go down..

I'm wrong but why?

mictester · Post by **mictester** » 29 Jul 2011, 10:44

Why don't you get a 40106 (it'll cost a few pennies), get the capacitors and the resistors and an LED, and try it out? Get your multimeter and connect it to the inputs and outputs of the inverters and you'll quickly see what's going on!

Fender3D · Post by **Fender3D** » 29 Jul 2011, 11:08

YuGi wrote:Example with 9v power supply, as I understand the inverter input is 1 (4.5 V) then the output is 0 (0v): the LED is off and the capacitor is discharged.
When I press the switch the capacitor charges because is connected to 4,5v and the voltage at the inverter input go down for a moment inverting the output status (9v). At this moment the LED is ON but if I hold pressed the switch the capacitor charges until 9v, the inverter input "sees" 9v and the output go down..
I'm wrong but why?

Yep but being the 100k smaller than 1M, it will keep its state nevertheless

bubstance · Post by **bubstance** » 30 Jul 2011, 18:33

earthtonesaudio wrote: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.

Now just to be clear, you mean wiring a 555 something like this, right?

DeanM · Post by **DeanM** » 01 Aug 2011, 09:11

Mictester, I simulated the circuit in LTSpice. In the on state it does indeed draw low current @ around 60uA in the simulation, as was also measured by someone in this thread IIRC. In the on state its the 150K resistor thats drawing most of that current. The coil only draws current when switching.but it momentarily draws around 30mA. From looking at datasheets for those gates you mentioned, the most i have seen, for the 40106, was up to 10mA output current. Have you found that to be enough current to drive the coil? From looking at the simulation and data sheets it seems like it would be a bit low?

Also, I combined the two schematics. Can you confirm this is the correct configuration that you use?
NOTE: Relay wasn't in the eagle library so i put a random one for now! relay should be Takamisawa AL5WNK. Power pins not shown either.

mictester · Post by **mictester** » 01 Aug 2011, 19:32

Dean - that'll work OK. If you're really concerned about the momentary current spike as the capacitor charges, you can parallel two (or more) of the gates for IC1C. However, I've built loads of them and never had a gate fail. It's useful to have two sets of three gates in the IC, because you can easily build two complete switching systems with just one IC - useful for the Fuzz Face driven by a Treble Booster configuration that's become very popular around here lately (it gives Fuzz Face, Top Boost or pseudo-Tonebender modes with two footswitches!)

DeanM · Post by **DeanM** » 01 Aug 2011, 20:51

mictester wrote:Dean - that'll work OK. If you're really concerned about the momentary current spike as the capacitor charges, you can parallel two (or more) of the gates for IC1C. However, I've built loads of them and never had a gate fail. It's useful to have two sets of three gates in the IC, because you can easily build two complete switching systems with just one IC - useful for the Fuzz Face driven by a Treble Booster configuration that's become very popular around here lately (it gives Fuzz Face, Top Boost or pseudo-Tonebender modes with two footswitches!)

Cool thanks for the confirmation. Well to be honest i was thinking more in terms of it being enough to drive the coil rather than the risk of damagine the CMOS! but that makes sense to parallel the unused gates for extra protection. but as you said, its worked so far so shouldn't be a problem.

True! in a dual pedal it would make good use of all gates! Its actually pretty convenient there's six inverters and 3 per switching circuit! haha!

Thanks for sharing!

bubstance · Post by **bubstance** » 06 Aug 2011, 20:49

Now here's a question I've been particularly curious about: is there any way to translate this to mono in/stereo out effects?

mictester · Post by **mictester** » 06 Aug 2011, 21:27

bubstance wrote:Now here's a question I've been particularly curious about: is there any way to translate this to mono in/stereo out effects?

Yes. Get two of the relays. Wire their coils in parallel. Increase the 100µ - I used 220µ (though you might still be able to get away with 100µ) to give a higher current pulse. You've then got QPDT switching. Should be enough for stereo out!

bubstance · Post by **bubstance** » 06 Aug 2011, 21:29

Awesome. Thanks a bunch!

rasta_maleek · Post by **rasta_maleek** » 01 Sep 2011, 11:20

what could i do to use the same relay, but 9v type?

Cub · Post by **Cub** » 01 Sep 2011, 11:35

I've been thinking of doing something with relays so I could use the four tip to ground switches on a Mesa/Boogie TriAxis to control a couple of stompboxes and this might be what the doctor ordered. I thought about ordering the "clickless true bypass" thing, but that would be US$ 120,- just for the four boards with relays and some other little parts.
Thank you very much for sharing this !

theItalianDiYer · Post by **theItalianDiYer** » 11 Sep 2011, 18:28

hei mick if I run the circuit with a higher Vcc shall I change anything in the circuit? the electrolityc cap?

mictester · Post by **mictester** » 07 Dec 2011, 23:38

theItalianDiYer wrote:hei mick if I run the circuit with a higher Vcc shall I change anything in the circuit? the electrolityc cap?

What voltage are you thinking of using? The 5V relay won't like bigger current pulses than it already gets!

If you have higher voltage, lose some of it with a resistive potential divider. Remember - this circuit only draws current during switching, so you won't be wasting much energy!

mictester · Post by **mictester** » 16 Jan 2012, 01:00

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

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 47µF to 100µF, 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.

: Little bit of Veroboard

Remember - the relay MUST be a Takamisawa AL5WN-K. Experiment (and a component shortage!) showed that the capacitor can be as small as 10µF (though it will stop working when the battery gets below about 8.5V) - my latest builds have used 47µF 16V tantalum beads (because I've got a boxful of them). I find that I still have plenty of the "Carling" SPDT footswitches that were pulled from old pedals to use up!

mirtzbass · Post by **mirtzbass** » 01 Feb 2012, 08:14

hi! one question : can i use this system with a boss ds-1?? thanks

A Switching Scheme [documentation]