Need a transistor-less, IC-less photocell circuit

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Yerry
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Need a transistor-less, IC-less photocell circuit

Post by Yerry » Tue Nov 10, 2009 7:42 pm

Without naming the organization, a HS-level science organization has a Rube Goldberg machine event, and one of the steps is a real stumper:

Use current through a photocell to turn on a small motor. The motor must perform the next step (read: actually do something)

Here are the killer clauses:

10V max (assume 6 D cells,w hich means an actual voltage of 9V)
NO transistors, FETs, ICs, microprocessors.

Here's what my kids run into:

Photocells are good for miniscule current. Not an issue since. . .
The smallest motor we could find was a 1.5v, 60mA motor.
Lowest value (when illuminated) photocell had a resistance of 3Kohms.

A simple circuit of battery, photocell and motor with the values above would take TWENTY FOUR of these photocells in parallel! I suppose "some" of those photocells could be replaces with a resistor at a value low enough to simulate (and replace).

Clarifications have been requested, and pleading to make the step easier have fallen on ears that have no real experience with electronics. I'd like an idea do-able within these restrictions that ANY kid in High School can do-- as it stands, only a "dadbuilt" is going to be in a position to get all 10 steps (and win).

Yerry

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Re: Need a transistor-less, IC-less photocell circ

Post by Robert Reed » Tue Nov 10, 2009 8:56 pm

Yerry
The power companies Use a plug in device for turning on street lights at dusk. These use no semis as they pull in a small relay directly. I happen to have one or two of these and if you like I could check for part nos. As yet I have not delved into these gadgets due to always saving them for special projects but maybe I could open one up and check the PC's parameters.

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Re: Need a transistor-less, IC-less photocell circ

Post by jimmy101 » Wed Nov 11, 2009 7:10 am

Use current through a photocell to turn on a small motor. The motor must perform the next step

Is that the exact wording of the challenge? Or do you just have to use a photocell + motor to do the next step?

Could the photocell be used to stop an already running motor? That might be easier since you don't have to source the full current of the motor, including the big surge current it takes to get a stopped motor to actually start turning. You would just need to sink enough current to get the spinning motor to stall. I would suspect that that is much less current than the 60mA the motor you describe draws. Lets see, motor + resistor + battery in series. Tweak the resistor until the motor just barely keeps running and you have to spin it by hand to get it to actually start turning. Put the solar cell in parallel across the motor.

Small reed relays appear to take about as much power to operate their coil as does your DC motor, so that doesn't help.

Can you have the photocell turn off a small relay? Same trick as with stopping a motor, just need to sink enough current from the relay's coil driver to get the relay to switch off. As with the motor, supply more current to close the relay, then limit the current to just barely enough to keep the relay closed. Put the solar cell across the relay's coil. You would have to set the relay on before the machine is operated but I suspect there are a lot of things set before the machine operates. If you wire the motor to the relays NC contact then the motor will start when the relay switches off.

Is the limitation on the use of semiconductors true for the entire challenge? Can you use a semi that is not directly involved in the actually turning on of the motor? I'm thinking of using the flash circuit from a disposable camera. A single AA battery charges a 100uF cap up to ~300V. Use that to power the switching system instead of the 9~10V.

Can you charge up a cap before operating the machine?

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Bob Scott
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Re: Need a transistor-less, IC-less photocell circuit

Post by Bob Scott » Wed Nov 11, 2009 12:31 pm

Off the top of my head: Remove the plastic window from a voltmeter and wire it so that the voltmeter's pointer hits a homemade wire contact in the pointer's path to turn the motor on. The other switch contact is the voltmeter's "chassis" and conducts through the meter's coil spring.
Have a CDs photocell in series with the battery and a ~ 1K or 19K resistor. The meter senses the voltage across the resistor.

With no active devices, what is left for you to use but a sensitive relay? And what level of electronics do the instructors expect from a high school student except for batteries, resistors, galvanometers and Ohms's Law?

I suspect that these exercises are intended to be Dad-assisted, so that Dad gets involved in Junior's education. I remember my sons' projects building "machines" in Grade 4.
-=VA7KOR=- My solar system includes Pluto.

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Re: Need a transistor-less, IC-less photocell circuit

Post by sghioto » Wed Nov 11, 2009 1:53 pm

As Bob suggested a sensitive relay might work. A 5 volt, 500 ohm coil reed relay is available from All Electronics (CAT# RLY-515) which might work using several photocells in parallel with a resistor to activate the relay. I would adjust the resistor value to set the voltage on the relay a volt or so below the pull-in threshold when the photocells are dark.
Steve G.
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Re: Need a transistor-less, IC-less photocell circuit

Post by fripster » Wed Nov 11, 2009 3:12 pm

mmm. no-one said anything about vacuum tubes :smile: so we can use a lowvoltage tube (some have 6V anode specs!).

just my two cents...

fripster
Once a WireHead, Always a WireHead

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Re: Need a transistor-less, IC-less photocell circuit

Post by jwax » Wed Nov 11, 2009 4:19 pm

What's the definition of a "photocell"? A solar cell fits the description according to wikipedia.
Use a photovoltaic cell/module.

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dacflyer
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Re: Need a transistor-less, IC-less photocell circuit

Post by dacflyer » Thu Nov 12, 2009 8:18 am

if its a Cds cell, i have seen some that are as low as 50 ohms when exposed to light.
and i have seen some cd spindle motors run on very low currents.
ya just have to dig in your junk box and hope for luck..

now if its a solar cell - then the options are easy.

just have to find out more info for us..
good luck..

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reloadron
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Re: Need a transistor-less, IC-less photocell circ

Post by reloadron » Thu Nov 12, 2009 11:33 am

Robert Reed wrote:Yerry
The power companies Use a plug in device for turning on street lights at dusk. These use no semis as they pull in a small relay directly. I happen to have one or two of these and if you like I could check for part nos. As yet I have not delved into these gadgets due to always saving them for special projects but maybe I could open one up and check the PC's parameters.
I would go that route. Sitting here at work I have a small relay labeled 300 Ohm 23 mA for the coil. I grabbed a random photocell from a junk box and at 6 VDC that little random photocell turns the relay on and off just fine. They don't say no relay's right? This would allow running a 6 VDC little hobby motor using the relay that is directly turned on by the photocell. Works sitting here on the bench with a few junk box parts. Ambient flourescent light and as I pass my hand over the photocell the relay de energizes and the photocell has no problem with the current draw.

<EDIT> With 6 VDC applied (relay & photo cell in series) the relay pulls in with about 3.5 volts applied and drops out at about 0.8 volts. The photo cell has NSL-5910 9851 written on it and the relay is a small can type with HI-G INC. written on it, 2BC-1B-107D 23mA DC 300 Ohm. For what it's worth. </EDIT>

Ron

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dacflyer
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Re: Need a transistor-less, IC-less photocell circuit

Post by dacflyer » Fri Nov 13, 2009 4:30 am

NOTE >> the older fotocells the street lights were and many still are using the cell and relay.
they are pretty robust, but prone to cycling if lightning is bright enough to flick the light off for a sec. then the light has to cool off before restriking..
they are being replaced with the electronic ones now days. they have a sensitivity delay so that the bright lightning will not mess with the lights cycling on / off so often. but i see a lot of the electronic ones fail here as well.
as a kid i used to use a pin spot lantern to shine on the foto cells and make them turn off,,
it impressed my friends, but annoyed the neighbors..lol lazer pointers work as well sometimes.

Robert Reed
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Re: Need a transistor-less, IC-less photocell circ

Post by Robert Reed » Fri Nov 13, 2009 7:54 am

Where did Yerry go?

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reloadron
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Re: Need a transistor-less, IC-less photocell circ

Post by reloadron » Fri Nov 13, 2009 8:26 am

Robert Reed wrote:Where did Yerry go?
Beats me? :???:

I thought we had some pretty good solutions here.

Ron

Yerry
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Re: Need a transistor-less, IC-less photocell circuit

Post by Yerry » Fri Nov 13, 2009 11:03 am

I'm wrapping up a two-day nature camp with 30 middle school kids.

Pray for me.

Yerry

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Lenp
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Re: Need a transistor-less, IC-less photocell circuit

Post by Lenp » Fri Nov 13, 2009 12:58 pm

FYI

Some of the common photocell controls use a simple thermal switch. A photo-resistance cell in series with a 2 watt resistor is wired accross the power and neutral. The resistor is mounted to a bimetal switch. In daylight the resistor heats, switch opens, lights off, and the inverse is true.

Primitive? Yes! Works? Yes! It also has an inherent time delay due to the thermal lag and it can switch big loads.
When you power it up in daylight the lights will come on for a short time until the bimetal heats. Self-test? Yes it has that too!

Hey, who need semiconductors???

Len
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reloadron
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Re: Need a transistor-less, IC-less photocell circuit

Post by reloadron » Fri Nov 13, 2009 2:54 pm

This is what I had working on the bench with what were basically junk box parts:

Image

I intentionally didn't connect the motor's (+) lead as I guess the user could decide if they wanted the motor running in light or dark. I actually Googled the relay and found a PDF file which can be found here. THen I Googled the Photo Cell and that PDF can be found here. Considering this is a HS level science project it might be interesting to start with about a mile of AWG 30 or 32 magnet wire and make a relay? Possibly the relay on and off could be modified or adjusted if the photocell was padded with a pot? I never tried it. Also the four D cells could be replaced maybe with a 6 Volt lantern battery. The photocell is readily available from most parts houses for about three bucks and I have no clue on the relay.

Years ago I needed to build my own form of analog opto coupler so I grabbed a sack of one of those 10 photo cells for a few bucks from Radio Shack because I needed something in a hurry. I think at least 5 of 10 in each package were the one I used. I have no clue where the relay came from but there were several in the junk draw all used as there were leads cut off the solder terminals. I would guess any similar relay would work just fine like a 5 volt relay from Radio Shack or as someone mentioned a single little reed relay. That or back to a mile of AWG 30 or 32 magnet wire and get creative.

Ron

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