Solid State relay question.

I am looking at buying a bunch of solid state relays to MIDI'fy an old keyboard / synth.

I was planning to use it across the key contacts of it's keyboard.

Anyway, I've sourced a bunch of "LH1547" solid state relays, but an a bit confused.

On the schematics, there is a diode across the S - S' junction.. but the blurb says that the "LH1547 is a SPST normally open unidirectional relay that can switch DC signals"...
Does this statement seem to contradict itself? Unidirectional & DC ? The diode ?

Can someone please clarify this for my poor self?

Kind regards
Mike

Oh, and keyboard schem.. see lower left for contacts.

I think these relays should work, but I'd like a second opinion.

Kind regards
Mike

On the schematics, there is a diode across the S - S' junction.. but the blurb says that the "LH1547 is a SPST normally open unidirectional relay that can switch DC signals"...
Does this statement seem to contradict itself? Unidirectional & DC ? The diode ?

This means that you have to watch the DC polarity. These can only be wired one way.

Way back in 1980, we had a shop layout the PC boards for our new product.  We spent a lot of time explaining all the details. Then after the first few boards, they switched personal. Yep, the new guy did all the relays backwards.

ahhh. I read "Unidirectional" but I was thinking something else.. lol.. Uni => 1 => 1 direction. I see.

why a ss relay?
why not a fet (switch)?

The keyboard is used as a voltage divider the key connects to one of the taps and goes to u31a

Now it looks like that voltage can be +8.24VDC at the right side of the keys and I am not sure what voltage at the left side of the keys
Check the voltages to ground at nodes, R74 and the left of the keyboard, R75 and the right side of the keyboard.

23 ohms in series with R77 2.2K is small might be fine

Hi Mike,

I guess it is a MOOG THE SOURCE?
Well, it might be lots easier to MIDI-fy it when you use a MIDI-CV/Gate-Interface as this synth has the connections for it on the back.
I did it with mine those days with  DOEPFER-Interfaces (a german brand).
Look here to find things to diy or buy as working units:

http://www.doepfer.de/home_e.htm

Hope to have helped,
best,

Udo.

> that voltage can be +8.24VDC at the right side of the keys and I am not sure what voltage at the left side of the keys

Opamp, other input at ground. Easy to show that the inside end of R74 is zero +/-5mV, and the first contact is about 0.33V.

i.e. all switch contacts are between zero and +8.3V.

N-JFETs with gate diodes swung +10V for ON and -5V for OFF will work.

4066 powered with zero and +10V will work. Better if a -1V supply can be rigged. The 4066 will need low and >+7V logic input. 4051 has logic level converters, but is only convenient IF your data is already 6-bit binary not 1-of-36. Dunno what comes out of a MIDI interface.

The ~~200 ohm resistance against R76 220K will cause a small voltage error added to the existing error of 220K against 0-37.5K keyboard divider.

JFETs or CMOS needs "no" control power, whereas the suggested opto-switches need considerable LED power. Weaker CPUs needed buffers, though I suppose all recent ones will pull the several mA needed to light the LEDs.

> MOOG THE SOURCE? .. use a MIDI-CV/Gate-Interface as this synth has the connections for it...

If that works, it makes a LOT more sense than hacking the keyboard.

kante1603 said:

Hi Mike,

I guess it is a MOOG THE SOURCE?
Well, it might be lots easier to MIDI-fy it when you use a MIDI-CV/Gate-Interface as this synth has the connections for it on the back.

Click to expand...

Cheers for the input, Ideally I'd want to try and avoid a Midi-CV.
I don't like the fact I can't play it when the CV& Gates are connected etc.. I want to be able to play over the top of sequenced material, if that makes sense.

I realise that this way is a bit backwards..  but I think it'll work for me better in the long run.

I was planning to use 2 of these
http://highlyliquid.com/midi-decoders/md24/

MIDI-> 24x 5v Logics. (Programmable etc )

I've used these in the past to MIDI'fy a few old drum machines. Apparently they are able to drive reed relays, so I'd imagine that they should be fine with the SS relays internal LED, but I'm prepared to be proven wrong.

As always, your input is valuable.
Cheers
Mike

PRR said:

> that voltage can be +8.24VDC at the right side of the keys and I am not sure what voltage at the left side of the keys

Opamp, other input at ground. Easy to show that the inside end of R74 is zero +/-5mV, and the first contact is about 0.33V.

i.e. all switch contacts are between zero and +8.3V.

Click to expand...

Cool, I was a bit concerned about that.. so in theory, the SS relays 'would' work, even though it's not the most elegant solution?

I could not read the symbol at pin 3 of u30a clearly, looks like it has an A inside the ground like symbol, so I did not assume it was ground(pin 2 and pin 3 should be the same voltage to ground).  That is why I posted to measure the voltages in case there was an offset.

3290a is a CMOS comparator.  It looks like the C18,R76, R77 are an RC network that develops a different pulse width because of the different taps for each key that signal then goes to U32 a TTL buffer chip

> in theory, the SS relays 'would' work

I'm going to say "no". The diodes will conduct. Imagine hitting note 24, voltage about 4V. If you put the SSRs one way, 23 diodes conduct to the lower voltages. If you put them the other way, they conduct to the higher voltages.

You can't use this type "unidirectional switch with reverse diode".

> 3290a is a CMOS comparator.

Thanks!! In "my" world, that would be a precision opamp buffer and ultimately go to an exponential oscillator. Clearly this is something else, because as you say it feeds a TTL buffer......

> It looks like the C18,R76, R77 are an RC network that develops a different pulse width because of the different taps for each key that signal

The string of identical small resistors is the standard way to scale the keyboard for an exponential synth. The voltage comes out equal-tempered; the volts/octave can be adjusted with the "8.24V". So the voltage near R77 is the desired pitch.

But we don't take that voltage directly. Instead I think the key is "KYBD APPROX". Some off-board gizmo watches U31A output and slews a voltage up/down until U31A is on the edge. Some form of voltage-matcher, and very likely an A/D converter. The D side of that also controls oscillator pitch somehow. Having pitch in digital form allows several/many more tricks.

Note also that there is no separate "Key Pressed" contact or pick-off. Whatever is watching U31B can detect the rise from zero to >0.3V and know a key is pressed.

> looks like it has an A inside the ground like symbol

I think that's just "analog ground". We have a mess of TTL near here, TTL ground glitching will glitch-up the pitch voltage, it would be good practice to run separate ground for analog.