Light sensitive relay switch alternative?

Yes, make the capacitor at + input of opamp smaller for a smaller delay.

Cheers! I've noticed that it takes longer to switch one way than back again. Also, I notice that there's two capacitors, one 100uf and one

470uf. Is this implementing the two different delays? What would happen if I took the capacitors off and just wired direct? Would that give me no delay or would it stop working or blow something?

I apologise for not knowing too much about what must be GCSE level electronics. I do very much regret opting for a different subject at school..!

andyt

I've just noticed that my problem is almost the exact opposite of the one that Nikki has in the post "Time delay using relays". Spooky! And that seems to be a similarly crackpot scheme too!

andyt

Well it sure as hell wasn't accounting- you're using about $350 worth of needless stuff....

heh, well I obviously don't know how to design it more efficiently. If you've got suggestions, I'm all ears!

andyt

Do notice: "CAPACITOR AT + INPUT OF OPAMP"

The other capacitors, according to circuit diagram at the webpage selling the circuit, are just for powersupply.

Schools are just for realizing that by reading books you can find out ANY information. For learning electronics, find a book on electronics, read it and start doing stuff. You can test circuits with free switcher cad 3 download:

Most simple electronics, like this circuit, are very easy to design - it is just resistive voltage dividers and basic stuff like that.

One resistive divider, 100k potentiometer, gives the comparator switch level to - input.

Another resistive divider 150k, VDR, 15k gives the other voltage. There is, in addition, a "delay" circuit by the 17k and 180k resistors, diode and 100uF capasitor.

When LDR has high resistance (dark), voltage is high and the

100uF capacitor charges slowly to this high voltage (and some current goes through 100k but to simplify forget this now). When LDR resistance drops (light), diode prevents the lower voltage from charging the capasitor, and capasitor discharges through the 100k resistor until eventually voltage is lower than the voltage set by the 100k potentiometer, and the state of the relay changes.

Now, when LDR again has high resistance (dark), the 100uF cap is starting to charge and voltage eventually becomes higher than the voltage from 100k pot, and the state of relay changes again.

The opamp (the triangle part with inputs marked "-" and "+" is a comparator - it compares whether - or + input has a higher voltage, and changes output accordingly.

LDR = light dependant resistor

I hope this helps you understand how the circuit works. You can enter the circuit to switcher cad 3, and replace LDR with a switch.. See how it works!

To answer your original question: Only the 100uF capacitor matters for the delay. Take it away and you have no delay. I'd suggest setting a little delay. Simulate the circuit with Switcher cad 3, and find out correct size for cap!!

How is this "laser harp" supposed to work activation-wise? Do you play it or does wind blow through it or something else? How do you simulate pressing a key on the MIDI kbd? Is the key grounded or how is that working?

It's just light triggers, so there's a bunch of beams shining above, down to a row of light sensitive switches and as soon as your hand breaks the beam and stops the laser reaching the LDR, it triggers the key. When you move your hand to allow the beam through, the key is released.

That I haven't tried yet (today's experiment was simply wired to a multimeter set to buzz on connection). The idea is to buy a very simple MIDI controller keyboard (one that isn't velocity sensitive), which hopefully will have simple contact switches for key down/up. I'll hook each relay into a key on the keyboard. The keyboard will then think the hay has been pressed or released and send out the appropriate MIDI signal to a sound module.

This is perhaps the design of someone who doesn't actually know electronics hence the use of an actual keyboard when perhaps some latch/MIDI convertor board would do..! However, I figure a keyboard will give me octave shifting (small keyboards often feature this), which might be useful when I set the sound module to a drum machine (which usually starts at key C1), or to a piano, where I would want a higher octave.

andyt

Ah! Ok, cheers.

OK, I'm gonna play with that. I do want to learn this stuff and with an actual project to push me into learning it, it should be the motivation I need.

Thanks for that. I kinda understand it, but only in that way where you understand what someone's saying, but you couldn't write it yourself (if you see what I mean!). I'll play with that software and perhaps figure out for myself what changes I want to make. At least any mistakes won't result in real damage or mishaps!

I'll do that, thanks. And I appreciate your help with the bigger picture, rather than just telling me what to do, but helping me figure it out for myself. Teach a man to fish and all that...

Thanks again.

andyt