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Electronics advice

The relay switch uses the force produced by an electromagnet to close two (or more) contacts together, or open two (or more) contacts that were closed before current was applied to the coil of the electromagnet.

I think the idea in the link uses the current normally used to drive a speaker to drive the relay coil, thus opening or closing the appropriate connections on the camera to trigger it to take a photo.

What I don't understand about the idea (as presented, there may be diodes and stuff not shown) is that relays usually work off direct current (DC), but speakers are driven with alternating current (AC).

Applying AC to a relay usually results (at best) in a buzzing sound, with no switching taking place.

One way around this would be to put a diode in series with the relay coil, thus converting the AC to pulsating DC, though the resulting voltage drop may mean a relay with a very low voltage / current coil is required. Additionally, it is recommended to connect a diode in parallel with the coil in the opposite direction to the one in series it.

Hope that helps.
 
Ivor the Engineer said:
The relay switch uses the force produced by an electromagnet to close two (or more) contacts together, or open two (or more) contacts that were closed before current was applied to the coil of the electromagnet.

I think the idea in the link uses the current normally used to drive a speaker to drive the relay coil, thus opening or closing the appropriate connections on the camera to trigger it to take a photo.

What I don't understand about the idea (as presented, there may be diodes and stuff not shown) is that relays usually work off direct current (DC), but speakers are driven with alternating current (AC).

Applying AC to a relay usually results (at best) in a buzzing sound, with no switching taking place.

One way around this would be to put a diode in series with the relay coil, thus converting the AC to pulsating DC, though the resulting voltage drop may mean a relay with a very low voltage / current coil is required. Additionally, it is recommended to connect a diode in parallel with the coil in the opposite direction to the one in series it.

Hope that helps.
Click to expand...

Thanks. I think I get the general gist. What does the extra diode do?
 
When current is flowing through a coil, it "wants" to keep flowing. When there is no electrical circuit for the current to flow around, the collapsing magnetic field produces a large (100's volts) spike. The extra diode provides a path for the current to flow when the circuit is interrupted, thus stopping the generation of the voltage spike, which can often damage the circuit driving the coil if not suppressed.
 
Ivor the Engineer said:
When current is flowing through a coil, it "wants" to keep flowing. When there is no electrical circuit for the current to flow around, the collapsing magnetic field produces a large (100's volts) spike. The extra diode provides a path for the current to flow when the circuit is interrupted, thus stopping the generation of the voltage spike, which can often damage the circuit driving the coil if not suppressed.
Click to expand...

I can so vouch for that!

We had one of the surge suppression diodes go out (opened) and every time the relay cycled it caused another circuit board, in a totally different drawer in the equipment rack, to blow (messed with the power supply on the 2nd board, basically).

This was when I was working on inertial navigation equipment on submarines. Luckily we were in port at the time, doing upkeep. Had the manufacturer's reps on board working on the problem, and it took them days, and a stack of the 2nd boards before they figured out what the problem was.

I'd probably have never found it.
 
Ivor the Engineer said:
The relay switch uses the force produced by an electromagnet to close two (or more) contacts together, or open two (or more) contacts that were closed before current was applied to the coil of the electromagnet.

I think the idea in the link uses the current normally used to drive a speaker to drive the relay coil, thus opening or closing the appropriate connections on the camera to trigger it to take a photo.

What I don't understand about the idea (as presented, there may be diodes and stuff not shown) is that relays usually work off direct current (DC), but speakers are driven with alternating current (AC).

Applying AC to a relay usually results (at best) in a buzzing sound, with no switching taking place.
One way around this would be to put a diode in series with the relay coil, thus converting the AC to pulsating DC, though the resulting voltage drop may mean a relay with a very low voltage / current coil is required. Additionally, it is recommended to connect a diode in parallel with the coil in the opposite direction to the one in series it.

Hope that helps.
Click to expand...
The buzzing is caused by the contacts snapping back in forth in response to the fluctuating current. The relay switches on and off very quickly. If the debounce for the camera shutter release is short enough, it'll fire on every click in the buzz. If it is longer, it'll fire every time the period between buzz clicks is long enough.

The guy also mentions that he gets multiple shots on each activation - to be expected the way he's going at it.

To get a good, single output from the "ring" feature of his radios, he'd need to rectify the speaker signal, then filter it to make on clean output signal from the audio.

It wouldn't take anything sophisticated. A transistor with a resistor between the base and the speaker output would do. The Base/Collector junction rectifies the signal, so that's a freebie. Connect a small capacitor from the Base/Resistor connection to ground, and you have a filter. Tweak for reliability (resistor and or capacitor values depending on what's in you junk box) and you'll be good to go.

Like this:
shutter.jpg

The given values ought to be close enough to be useful.
 
Ivor the Engineer said:
When current is flowing through a coil, it "wants" to keep flowing. When there is no electrical circuit for the current to flow around, the collapsing magnetic field produces a large (100's volts) spike. The extra diode provides a path for the current to flow when the circuit is interrupted, thus stopping the generation of the voltage spike, which can often damage the circuit driving the coil if not suppressed.
Click to expand...

Sort of. When you stop feeding current into the coil, the magnetic field collapses. The collapse induces current flow in the conductor of the coil, but in the opposite polarity of the current you were feeding into the coil. If the current can't flow, the voltage builds up trying to force a current flow. At some point, it probably will succeed in pushing current backwards over some semiconductor's reverse breakdown voltage. That tends to ruin a semiconductor's day, and they burn out.

Putting the reverse polarity diode across the relay limits the voltage that the coil can reach to 0.7V, and it gives the current a low resistance short circuit to dissipate into.
 
MortFurd,

I think you need to slightly rethink your circuit arrangement.

The speaker driver outputs a tone, which will be an AC signal with zero DC level. The RC low-pass filter on the base of the transistor in your circuit will make AC signals at the base smaller, possibly stopping the transistor turning on at all! If you added a diode in series with the R, that should work.

Also, it is unknown which way current has to flow between the terminals on the camera, which would need to be determined if using a unidirectional device such as a bipolar transistor.

A relay provides electrical isolation between the radio and the camera, thus the chance of one device being damaged from a fault in the other (or the bit in between) is greatly reduced.
 
Not damaging the camera is quite important! :)

Working from Ivor's design, I assume that any spiking is more likely to damage the radio than the camera?
If I used an earphone lead with the radio and attached the relay to that, would it help protect the radio ( as well as meaning I can use it in different systems)?

Would that affect the design any?

Thanks for all the help so far!
 
Ivor the Engineer said:
MortFurd,

I think you need to slightly rethink your circuit arrangement.

The speaker driver outputs a tone, which will be an AC signal with zero DC level. The RC low-pass filter on the base of the transistor in your circuit will make AC signals at the base smaller, possibly stopping the transistor turning on at all! If you added a diode in series with the R, that should work.

Also, it is unknown which way current has to flow between the terminals on the camera, which would need to be determined if using a unidirectional device such as a bipolar transistor.

A relay provides electrical isolation between the radio and the camera, thus the chance of one device being damaged from a fault in the other (or the bit in between) is greatly reduced.
Click to expand...
What? Re-think a circuit I've used many a time, and which never failed?

The terminal marked "speaker" goes to the speaker. The ground terminal goes to the electrical ground of the radio. The AC coming on the speaker line will have peaks higher than the 0.7 volts needed to switch the transistor. The cap just smooths them, and basically prevents the DC level from dropping between pulses.

The output is not terribly critical. The camera input will normally have one side grounded and the other "hot." The ground is (normally) on the shield part of a 1/8 inch plug, so no problem there - and if you get it backwards, no sweat, just reverse it.

Adding another diode just increases the voltage drop, but it doesn't help the circuit in any way.

In the circuit provided, you have the ground lines of two (battery powered) devices connected. The only real source of current has (at a minimum) 100KOhms between the source and the camera. Safe enough.

Build one and try it. You may have to fiddle with the R/C combination to get the debounce right (depending on tone frequency and duration,) but the basic circuit is sound and functional.

Have you ever done anything along these lines?
 
sphenisc said:
Not damaging the camera is quite important! :)

Working from Ivor's design, I assume that any spiking is more likely to damage the radio than the camera?
If I used an earphone lead with the radio and attached the relay to that, would it help protect the radio ( as well as meaning I can use it in different systems)?

Would that affect the design any?

Thanks for all the help so far!
Click to expand...
Feeding across the speaker like it would, there's really no need for the reverse protection diode. The induced current/voltage from the relay coil (if you use a low current relay - both small and cheap) would short back through the speaker. No problem.

The problem will be that you could (and probably will) get more than one photo each time you signal the radio. This will happen because the relay switches on every pulse of the tone. You won't get a picture for every pulse of the tone, but you'll surely get more than one (unless the camera debounce is longer than the tone burst.)

Making the relay only trigger once is hard. You can't really filter the signal since you need lots of current (compared to the transistor) to switch the relay. You could try to do so with a resistor/capacitor combination like in the schematic I provided, but it would be low resistance and high capacitance.

If it were me, I'd build the transistor variant, and tweak the resistor and capacitor for most reliable single shot triggering. And I'd probably thread the cable to the camera through a ferrite bead to block RF.
 
Replace the DC reed relay with an AC reed relay, such as the DigiKey BE05-2A88-P-ND.

Such relays have more windings and greater hysteresis to handle the zero-point crossover of an AC signal. The N.O. contacts of the relay should still handle the camera's trigger current.
 
Fnord,

I've just learnt what a relay switch does - this talk of hysteresis and N.O contacts sounds like some kind of spiritualist woo... :)

Could you explain it a bit.

Cheers
 
To keep the numbers simple, let's say the input to the circuit is a square wave:

When the input signal is +ve, the junction of the RC network is clamped at +0.7V by the transistor's base-emitter junction, turning the transistor on.

When the input signal is -ve, the junction of the RC network ramps down, below +0.7V turning the transistor off.

That assumes that the tone is of a sufficiently low frequency (i.e. the +ve half is long enough in duration) that the magnitude of the signal at the junction of the RC network reaches +0.7V.

By adding a series diode, the circuit becomes an envelope detector, remaining on for the duration of the tone. I actually missed something first time round - the base of the transistor needs a series resistor between it and the Diode-RC network; just adding a series diode will not help because the base-emitter junction would still clamp the voltage on the junction of the RC network to +0.7V, which would drop below this level when the diode became reversed biased, turning the transistor off.

As for values: The R in the RC low-pass filter ~10k, the C ~470nF, the R between the RC low-pass filter and the base of the transistor ~100k and the series diode could be an 1N4148 or similar.

This allows the junction of the RC low-pass filter to reach more than +0.7V when the input signal is +ve, thus allowing the transistor to stay on when the input signal goes -ve.
 
Fnord said:
Replace the DC reed relay with an AC reed relay, such as the DigiKey BE05-2A88-P-ND.

Such relays have more windings and greater hysteresis to handle the zero-point crossover of an AC signal. The N.O. contacts of the relay should still handle the camera's trigger current.
Click to expand...

That would work better than a DC relay so long as the frequency of the tone is not too high.

The contact bounce MordFurd mentioned might be an issue if the input is not debounced in the camera. I'd have thought it would be though, since push-switches circuits other people use appear to work.
 
sphenisc said:
Fnord,

I've just learnt what a relay switch does - this talk of hysteresis and N.O contacts sounds like some kind of spiritualist woo... :)

Could you explain it a bit.

Cheers
Click to expand...

Hysteresis: The lagging of an effect behind its cause. In this case, the turning off of the relay lags behind the applied voltage enough that the relay does not "chatter" when AC is applied.

N.O. Contact: Relay or switch contacts that are normally open; that is, they do not touch each other when power is removed.

Fnord's Law: Any previously-unlearned technology is dismissed as woo.
 
Here's a simulation of the diagram I provided:
ivor.oregano.jpg
Red is the speaker signal. Green is at the connection of the resistor and the capacitor, and white is the camera shutter signal.

Provides a single pulse deep enough to trigger the camera, the following little bumps are too shallow. That's with the values I posted. A little tweaking could minimize the bumps even more.

Increasing the capacitor to 0.15microfarad gets rid of the bumps. Adding a 1microfarad capacitor across the "camera" leads widens the pulse to about 20 milliseconds - which might not be needed, it depends on the camera.
 
Another simulation, with the above mentioned improvements:
ivor2.jpg

Both of the sims were done with the tone from the speaker at 100Hz. for higher frequencies, scale the capacitor inverse linearly with the frequency - for 400 Hz, use ~ 0.04 µF for the capacitor. The width of the output pulse depends on the frequency of the tone, so you might have to add the pulse widening capacitor to the output. Its value depends on the impedance of the camera input. A 1 µF across the camera leads gives the shown pulse width if the camera impedance is 10KOhm.
 
MortFurd said:
Here's a simulation of the diagram I provided:
View attachment 8620
Red is the speaker signal. Green is at the connection of the resistor and the capacitor, and white is the camera shutter signal.

Provides a single pulse deep enough to trigger the camera, the following little bumps are too shallow. That's with the values I posted. A little tweaking could minimize the bumps even more.

Increasing the capacitor to 0.15microfarad gets rid of the bumps. Adding a 1microfarad capacitor across the "camera" leads widens the pulse to about 20 milliseconds - which might not be needed, it depends on the camera.
Click to expand...

That is so nasty. Do you design stuff for Amstrad?
 
Final circuit:
ivor3.png


Simple enough. Use a ceramic capacitor for the 0.04µF, anything you can get your hands on for the 1µF.

Yes, I know. There are 2 C1 parts in the diagram. Sue me for not changing the part names. :)
 
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