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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 19th Aug 2018, 9:39 am   #1
Biggles
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Default Light activated SCR

About forty years ago I remember buying a component in Tandy which was described as a light activated silicon controlled rectifier. It came with the usual application circuit which was for a "magic candle" where the SCR controlled a small lamp (the candle flame) which the photo sensitive window could see. I think it was driven with AC from a small mains transformer. When a match was held to the sensor, the lamp lit, and this formed a loop with the photo sensor which kept the lamp on. To extinguish the candle, the flame was covered and this broke the loop and the "candle" went out. A novel project. In all the years between then and now, I have never seen a light activated SCR again. Have I got the only one? I still have it somewhere. I suppose I can see applications in process control and the like but they don't seem to have been very common.
Alan.
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Old 19th Aug 2018, 10:30 am   #2
Argus25
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Default Re: Light activated SCR

I have not seen that version with light feedback, but there is a better one I think. The trick is to use a small thermistor bead near/attached to the lamp. That way when you hold the match near the lamp it sets the bistable circuit with a delay. The heat from the lamp keeps it on after that (rather than light being used as the feedback mechanism) Then when you blow on the lamp, it cools the thermistor bead and it goes out, just like a real candle....
Probably a really good one could be made with a combination of a thermistor for blowing it out and an LDR or photo transistor for switching & holding it on.

Last edited by Argus25; 19th Aug 2018 at 10:40 am.
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Old 19th Aug 2018, 10:49 am   #3
GrahamN
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Default Re: Light activated SCR

I remember making a similar circuit using an OCP71 transistor (actually it was a glass bodied OC71 with the black paint rubbed off as that was much cheaper).
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Old 19th Aug 2018, 10:56 am   #4
Biggles
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Default Re: Light activated SCR

I think the light feedback circuit might have an inherent problem. I suspect bright daylight could trigger it. Without careful screening from ambient light it would probably play up. I like the thermistor version. Neat bit of lateral thinking there. I might even be tempted to try that out. A proper neon flicker type candle bulb probably wouldn't have enough heat output to work the thermistor but maybe a combination of the thermistor and a small incandescent bulb could be used to trigger a small triac or relay to light a neon candle bulb. Oh dear, I sense another project in the making!
Alan.
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Old 19th Aug 2018, 8:05 pm   #5
Ed_Dinning
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Default Re: Light activated SCR

Hi Alan, I have quite a few of these devices as opto couplers in IC form. We used them to trigger the large SCR's used in large DC variable speed drives.
There were a fussy device so it paid to carefully read the data sheet!!

Ed
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Old 19th Aug 2018, 8:58 pm   #6
Biggles
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Default Re: Light activated SCR

Cheers Ed. I thought they may have come from the industrial/process control side of things. It's quite a low power device in a BC109 type can (TO18?) so I presume it must have interfaced with a power device to do the job.
Alan.
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Old 21st Aug 2018, 12:22 pm   #7
dominicbeesley
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Default Re: Light activated SCR

That was one of the projects in my Radioshack 120-1 kit....I had to sneak the matches
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Old 21st Aug 2018, 6:31 pm   #8
60 oldjohn
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Default Re: Light activated SCR

There was a circuit in Practical wireless or Everyday Electronics 1970ish to 1976. It used OCP71 OC71 as Graham said.


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