Can you post your asc file, ebeowulf?
Nick, can you determine the approx minimum pulse duration needed to twitch the magnet when the coil is driven with 3V?

Sure, thanks!

I've included three versions to demonstrate what partially works and what appears to fail. I should say up front that the h bridge could/should probably be Darlingtons or MOSFETs, and that I paid no real attention to base resistor values, etc. on that side of things. I set out to see if the logic/timing of the basic concept worked before worrying about those details, especially since I was hoping to use some sort of h bridge IC that there wouldn't be a model for anyway. I had assumed that those details shouldn't prevent the basic logic/timing from being tested, but perhaps I was wrong on that.

 

Hi Ron_V, nothing at all, in fact I was going to order the components. I was just interested to see if it could be done any easier just using one coil.

Nick

Ahh, I thought I had missed something.
You might pick up a couple of smaller caps - 4.7 Ufd. in case you need shorter pulses.

 

Hi Ron_V, nothing at all, in fact I was going to order the components. I was just interested to see if it could be done any easier just using one coil.

Nick

The thing about using one rectangular magnet on a brass wheel is there is no need to change polarity on the coils. The offset weight of the magnet will always bring the pointer back to a "neutral" state/position. Since only one magnet pole is on the edge of the 'wheel' each coil would only need to be one polarity also, the opposite polarity of the permanent magnet on the 'wheel'. But according to the devises shown on the link given you need more than two 'stopped' positions to make this work. Unless only two "answers" are desired.

If the RC servo idea isn't workable for you, I do have another idea that is hard to explain but I can draw it up and post it if you want.

 

The thing about using one rectangular magnet on a brass wheel is there is no need to change polarity on the coils. The offset weight of the magnet will always bring the pointer back to a "neutral" state/position. Since only one magnet pole is on the edge of the 'wheel' each coil would only need to be one polarity also, the opposite polarity of the permanent magnet on the 'wheel'. But according to the devises shown on the link given you need more than two 'stopped' positions to make this work. Unless only two "answers" are desired.

If the RC servo idea isn't workable for you, I do have another idea that is hard to explain but I can draw it up and post it if you want.

I think he is looking for silent operation.

 

I think he is looking for silent operation.

I'll draw up my other idea and post it this evening.

 

ebeowulf, I think the problem in the sim is that, when the delay is used, everything above Q5 collector is initially at 6V, so the D input of the flip-flop is held at 6V whenever the flip-flop gets clocked.

 

The thing about using one rectangular magnet on a brass wheel is there is no need to change polarity on the coils. The offset weight of the magnet will always bring the pointer back to a "neutral" state/position. Since only one magnet pole is on the edge of the 'wheel' each coil would only need to be one polarity also, the opposite polarity of the permanent magnet on the 'wheel'. But according to the devises shown on the link given you need more than two 'stopped' positions to make this work. Unless only two "answers" are desired.

If the RC servo idea isn't workable for you, I do have another idea that is hard to explain but I can draw it up and post it if you want.

Hi Shortbus, I was considering two alternative scenarios. One was to have the pointer stop at either yes or no, the other was to have it stop at a random position. The random option is now my preferred route.

I don't want to use servos or motors of any kind as the mechanism has to be completely silent. Your other idea sounds intriguing, if you'd be so kind to post it that would be much appreciated.

Many thanks
Nick

 

I think he is looking for silent operation.

Thanks Ron_v, that's correct.

 

OK, here is my latest idea for you. But now you've thrown in another stipulation. Unless I missed it, what was shown on your link, the pointer has to stop at certain places so the message can be read. Correct?

Well here goes, remember it is just a rough sketch no dimensions or anything like that. Just a 'think out loud' idea. Each coil is wired in parallel with the positive battery terminal. Each other coil lead needs it's own switch. I showed four 'answers', but that's up to you. After reading the answer and turning off the switch, the unbalance will allow the pointer to swing to both sides of resting place before stopping. The stop position could also be labeled as "ready for next question", or not.

The nails in the center of the coil is needed, doesn't have to be very big. Don't know the terms in UK for nails, here it would be a 6 or 8 'penny' size. Just something to make the electromagnet force more concentrated. The permanent magnet has been replaced by a small piece of mild steel. If using a permanent magnet it would stop the pointer at each nail head and not return to the resting point.

 

OK, here is my latest idea for you. But now you've thrown in another stipulation. Unless I missed it, what was shown on your link, the pointer has to stop at certain places so the message can be read. Correct?

Well here goes, remember it is just a rough sketch no dimensions or anything like that. Just a 'think out loud' idea. Each coil is wired in parallel with the positive battery terminal. Each other coil lead needs it's own switch. I showed four 'answers', but that's up to you. After reading the answer and turning off the switch, the unbalance will allow the pointer to swing to both sides of resting place before stopping. The stop position could also be labeled as "ready for next question", or not.

The nails in the center of the coil is needed, doesn't have to be very big. Don't know the terms in UK for nails, here it would be a 6 or 8 'penny' size. Just something to make the electromagnet force more concentrated. The permanent magnet has been replaced by a small piece of mild steel. If using a permanent magnet it would stop the pointer at each nail head and not return to the resting point. View attachment 100298

Hi Shortbus, many thanks for posting that. It's an interesting idea but I'm not sure it's right for what I need. I don't want to use more than two coils and adding steel or iron to the cores would get really fiddly. Also, the small coils are just not strong enough to attract steel, hence why I'm using neo magnets. I'm more than happy with the performance from these coils, even at 1.5v - I just need to find a way of alternating between them activated by a single switch. I will be experimenting with a brass circular disc and small offset magnet - your suggestion there was spot on.

Thanks
Nick

 

You do understand that with only two coils there will only be two stopped positions possible. And the motion will just be like a pendulum in a clock.

 

You do understand that with only two coils there will only be two stopped positions possible. And the motion will just be like a pendulum in a clock.

I think the plan is to make the pulse short enough that the dial doesn't get to the position where it is held by the coil, so it just continues to spin.
The first coil pulls it towards it. If it's already there that's okay. Then it get pulled towards the second coil. Kind of a random thing.

 

If you wanted to try a single-coil version with pulse polarity reversal, here's my offering.
The switch would be provided by your RF receiver. A pulse of one polarity is generated when the switch closes and lasts for the shorter of ~0.5sec and the duration of switch closure. When the switch opens, a pulse of the opposite polarity is generated and lasts for ~0.5sec.

Standby current drain is zero (plus the RF receiver draw).

 

You do understand that with only two coils there will only be two stopped positions possible. And the motion will just be like a pendulum in a clock.

Hi Shortbus, Ron_v is right I don't really want any stopped positions, just for it to spin freely and come to a stop by itself.

Thanks
Nick

 

If you wanted to try a single-coil version with pulse polarity reversal, here's my offering.
The switch would be provided by your RF receiver. A pulse of one polarity is generated when the switch closes and lasts for the shorter of ~0.5sec and the duration of switch closure. When the switch opens, a pulse of the opposite polarity is generated and lasts for ~0.5sec.
View attachment 100327
Standby current drain is zero (plus the RF receiver draw).

Hi Alec_t, this looks like a pretty cool solution and fairly simple to build. Just a couple of questions if that's okay and appologies if they sound pretty basic.

1. Looking at the direction of the arrows on the schema, am I right in thinking one Mosfet needs to be N-channel and the other P-channel? If so please could you recommend a part number.
2. I assume I don't need a snubber diode across the coil? Given that we're reversing polarity it probably wouldn't work anyway - guess I've just answered my own question.
3. I've got some 10u caps that are 10v - would they be suitable for the circuit?

Many thanks, can't wait to breadboard this and give it a try.
Nick

 

If you wanted to try a single-coil version with pulse polarity reversal, here's my offering.
The switch would be provided by your RF receiver. A pulse of one polarity is generated when the switch closes and lasts for the shorter of ~0.5sec and the duration of switch closure. When the switch opens, a pulse of the opposite polarity is generated and lasts for ~0.5sec.
View attachment 100327
Standby current drain is zero (plus the RF receiver draw).

I like that! Clever use of the potential split supply with multiple batteries. Officially ditching my idea now! (Although I may still try to make it work just to learn what I've done wrong.)

 

If you wanted to try a single-coil version with pulse polarity reversal, here's my offering.
The switch would be provided by your RF receiver. A pulse of one polarity is generated when the switch closes and lasts for the shorter of ~0.5sec and the duration of switch closure. When the switch opens, a pulse of the opposite polarity is generated and lasts for ~0.5sec.
View attachment 100327
Standby current drain is zero (plus the RF receiver draw).

Looks good!

 

am I right in thinking one Mosfet needs to be N-channel and the other P-channel? If so please could you recommend a part number.

You're right.
I think you already have a PFET? (It needs to be one with a low Vgs(thr) value). As for NFETs, there are hundreds to choose from. Try a parametric search on the web-site of any stockist (e.g. Digikey, RS, Farnell). Look for something with a Vgs(thr) value <1.5V and a Rds(on) value <20mΩ.

I assume I don't need a snubber diode across the coil?

Simulation says not, since there's no significant voltage spike.

I've got some 10u caps that are 10v

They'll do nicely.

Edit: M1 is a PFET, M2 is a NFET.

 

@ronv and nickb, I'm sorry about that. What I got from reading the link provided was that the original devise was to spell out answers or point to answers. Like a vertical Ouija board. Not to spin like a roulette wheel.

 

You're right.
I think you already have a PFET? (It needs to be one with a low Vgs(thr) value). As for NFETs, there are hundreds to choose from. Try a parametric search on the web-site of any stockist (e.g. Digikey, RS, Farnell). Look for something with a Vgs(thr) value <1.5V and a Rds(on) value <20mΩ.

Simulation says not, since there's no significant voltage spike.

They'll do nicely.

Edit: M1 is a PFET, M2 is a NFET.

Hi Alec_t, many thanks for those answers, that's great. Ordering components today.

On another note, I'm looking for someone to collaborate with on other similar projects on a mutually agreeable basis. Is this something you would be interested in? If so please email me: [MOD NOTE: e-mail address removed - send a PM]

Thanks
Nick