dusk switch adjustable sensitivity...

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dacflyer
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dusk switch adjustable sensitivity...

Post by dacflyer » Mon Aug 05, 2002 2:31 pm

i am in a DUH ! moment,,,
i am tinkering with a photo-dioad & transistor set up & a pot.
i want the photo -dioad to turn on the transistor to give a hi out put when the photo-dioad is dark,
but my problem is,, as it gets dark the transistor slowly comes on,,, i do not want it to slowly come on,, i want either off or on,, no variance...i was thinking to use also a SCR, but when the SCR is triggered,,, i cannot figgure a way for it to turn off and be reset for next dark cycle....can anyone show me a diagram to use??
i am thinking its gotta be fairly simple,,,,
12v in - photo-dioad - adjustment sensitivity pot-
and ( transistor / SCR ) then 12v out....when dark,,, reply if not sure about this...i need it to control about 1 amp,,,thanx

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Edd
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Re: dusk switch adjustable sensitivity...

Post by Edd » Tue Aug 06, 2002 12:27 am

Dacflyer:
Seems like your going to have to move to a different circuit technique to afford the higher gain that is needed to get Ur desired result. This circuit is configured as a "psuedo" comparator circuit and is utilizing a photo-resistive detector element. Not knowing the characteristics of Ur photo "dioad" device, and whether it exhibits some photo-resistive characteristics, some of which I have known to do that, would neccessitate some experimentation in proper implementation into the -(pin 2) of the 741. Specifically as to whether it can merely function as a photo-voltaic unit or possibly exhibits equally acceptable photoresistive qualities. This would ascertain the polarity of installation into this specific circuit. Also you might have to swap the places of the photo device and the R1/Vr1 resistive elements.
This circuit shud provide your rapidly defined switching threshold that U desired. If you require an extreeemely sharp sw threshold, utilize a LM311 comparator, specifically designed for an application like this. And your circuit is:
http://www.mitedu.freeserve.co.uk/Circu ... darksw.htm <p>73's de Edd<p>[email protected] (Interstellar~~~~Warp~~~~Speed)
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russlk
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Re: dusk switch adjustable sensitivity...

Post by russlk » Tue Aug 06, 2002 4:21 pm

A Cadmium Sulphide photo resistor, transistor and relay may be easier to control and will switch on or off.

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dacflyer
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Re: dusk switch adjustable sensitivity...

Post by dacflyer » Tue Aug 06, 2002 6:38 pm

i tried that... its still comes on gradually rather than instant on or instant off...this is what i am trying to acheive.....and i do not want to use a relay...

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Re: dusk switch adjustable sensitivity...

Post by Isenbergdoug » Tue Aug 06, 2002 7:14 pm

I'm not quite sure that you will be able to get an on or off with the present set up. I suggest using a photo transitor in series with a variable resistor, or a CdS cell in series with a variable resistor, producing an adjustable voltage divider. Connect the common node of the comparator to a Schmitt trigger inverter like a 40106 which can handle the 12 volt supply. You can then take the output of the inverter and connect it to the base of your transistor via a biasing resistor. This set up not only allows you to adjust at what light level the system will change state, but with the hex inverter, you don't have to think about polarity of the signal as you can simply invert it if it is not right.

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dacflyer
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Re: dusk switch adjustable sensitivity...

Post by dacflyer » Fri Aug 09, 2002 1:24 pm

just an ammature here,, can you possibly draw me a schematic ? thanx...

Isenbergdoug
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Re: dusk switch adjustable sensitivity...

Post by Isenbergdoug » Sun Aug 11, 2002 6:58 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">code:</font><hr><pre> <pre><p>+12 0---/\/\---o---/\/\---0 GND
CdS | Var. Res.
|
|
---------
\ /
\ / Schmitt trigger inverter
+12 0----\ /----0 GND
\_/
(_)
|
|
|_________/\/\______
|b NPN
__|__
+12 o_____/ \___|-------|
c e |your |
|device |
|-------|
|
|
GND 0<p></pre><p>
</pre><hr></blockquote>
In order to change if the switch is on at dark or at light, you can either switch polarity on the voltage divider, or add another inverter in series with the present inverter.<p>[ August 11, 2002: Message edited by: Isenbergdoug ]</p>

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jollyrgr
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Re: dusk switch adjustable sensitivity...

Post by jollyrgr » Mon Aug 12, 2002 7:23 pm

You can get a very effective "snap action" light activation circuit by wiring two transistors as a "Darlington pair". See this link to get the schematic I will be referring to. Open in a new window.<p>http://electronickits.com/kit/complete/elec/ck1602.pdf<p>I have built a similar circuit to the one shown in this kit. I did not use the switch to change back and forth between a light detector and a dark detector. So you can elminate the DPDT switch and wire directly. I also used common NPN transistors such as 2N2222. If you use something like a 2N3055 you can easily replace the relay with your 1 amp load. Usually in this circuit you use two of the same transistor. You MIGHT be able to use a 2N2222 as Q1 and a 2N3055 as Q2. I have not tried this though.<p>[ August 12, 2002: Message edited by: Jolly Roger ]</p>
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Re: dusk switch adjustable sensitivity...

Post by bobsRAC » Thu Aug 22, 2002 10:39 pm

This seems like an ideal circuit for hysteresis. The use of a schmidt trigger would guarantee instant switch between on/off states. I much prefer to make a schmidt trigger from an opAmp than use on with preset hysteresis. This can be accomplished by using a resistor divider on the non-inverting input to the set the trip point and a larger resistor from the opAmp's output to the non-inverting input. This is positive feedback. The positive feedback acts to adjust the trip point based on the current state of the output.
The input is at the inverting terminal.<p>If the output is low (corresponding to a condition in which the signal applied to the inverting input is greater than the trip point) the trip point is decreased slightly because the feedback resistor is essentially in parallel with the lower half of the divider. This acts to reinforce the current state of the output.<p>When the output is high, the feedback resistor is essentially in parallel with the upper resistor in the divider, slightly increasing the trip point, again reinforcing the current state of the output.

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