light/dark sensor using LDR (need help)

[kamisochino]

Hi,

Can anyone verify my design for a light/ dark sensor i made (see attached image for the schematic)

Operation:

My design is a dual mode sensor. it could be set as a light sensor or a dark sensor.

SW1 would set the mode of the device. Flipping the switch would interchange the position of the resistors( R4+VR1) with the LDR. This would determine if its a light or a dark operated sensor.

SW2 would set the sub-mode of the device. Sub-mode 1 ( SW2 at C-B;E-F) would just be a plain sensor, output depends on what is sensed by the LDR. Sub-mode 2 (SW2 at C-A;E-D) would be a triggered sensor ( Continuous output once triggered regardless of the following conditions sensed by the LDR).

VR1 will determine the sensitivity of the device.

Thanks in advance .... any suggestions would be greatly appreciated.

Note: If the design would work pls help with what component names and values i would put on the final product tnx...

 

[(*steve*)]

I'm pretty sure that none of SW1, SW2, or VR1 will do what you want.

This page shows a light and dark sensor. Note that the LDR and the resistor need to change places to make the change. What is not obvious is that the resistor value will change considerably. The resistor can be a fixed resistor in series with a variable one to adjust sensitivity.

 

[kamisochino]

I'm pretty sure that none of SW1, SW2, or VR1 will do what you want.

This page shows a light and dark sensor. Note that the LDR and the resistor need to change places to make the change. What is not obvious is that the resistor value will change considerably. The resistor can be a fixed resistor in series with a variable one to adjust sensitivity.

i changed one connection of the LDR (see attachment). If I now flipped SW1 will it not still result to the circuits shown at the link you showed me?

thanks a lot for the reply by the way.

 

[davenn]

if you followed the circuits, in the link Steve cave, more closely. You mite start to get somewhere.
get rid of your SW2, get rid of the SCR for a start, this will simplify your circuit


Dave

 

[(*steve*)]

OK, that's a lot better. That part of the circuit will now probably work (although if you want this to be practical, you would probably want two potentiometers of different values, of for the "light sensitivity" and the other for the "dark sensitivity" -- but we can discuss that later.

Now let me try to figure out what the top part of your circuit is supposed to do...

OK, I see.

There are a couple of problems.

The first is that the LED current may not be sufficient to keep the SCR conducting (all this means is that you need an SCR that can operate at a low current)

The second, and more severe problem is that the SCR is triggered when the transistor is off, not when it is on.

If you look at this page, you'll notice that a connection between the gate and the anode turns the SCR on. This connection is provided by a fixed resistor in your circuit and relies on the transistor to inhibit it.

OK, this will work better:

Excuse the poor drawing around Q2. (which I forgot to label)

I have renamed all of your resistors so as not to confuse you (they all have different purposes now -- well most of them do)

R11 still sets the LED current.

R10 provides a pull-up of the collector of Q1 when the LED is not connected. a value of 10k should be OK

R12 limits current to the base of Q2. Again 10k should be fine.

R13 helps turn off Q2. I'd pick a value between 2k2 and 4k7.

R14 triggers the SCR when Q2 starts to switch off. Note that I show it connected to the anode of SCR1. I would recommend you connect it to the +ve supply rail. The value depends on the trigger current for the SCR. I'd start at 10k.

The operation of this circuit is reasonably straightforward.

With the switch in the position shown, the LED is turned on when Q1 is turned on. The SCR and the circuit around Q2 are effectively disconnected.

With the switch in the other position, the LED is in series with the SCR. it turns ON when the SCR is turned on. R14 turns on the SCR, but the gate is held low by Q2. When Q1 is off, Q2 is held ON by R10 and R12. As Q1 starts to turn on, the voltage at the collector of Q1 falls. When it falls sufficiently, Q2 turns off, allowing the SCR to turn on.

R13 is probably the best resistor to adjust to set the trigger point of the SCR with respect to what the LED does when S2 is in the other position. Making this resistor smaller (lower value) will increase the sensitivity of the SCR.

 

[kamisochino]

Wow! Thanks a lot for the great help (*steve*). Sorry to bother you with a simple problem. I'll start making it after I got all the components.

I'll just post back after testing it.

Thank a lot again.

 

[(*steve*)]

Before you go too far, the circuit is good enough to illustrate the concept, but it may be totally inappropriate in practice.

If you can give us some background on why you're building this and for what purpose then we may be able to steer you to a better solution.

 

[kamisochino]

Well its for a small school project. We were asked to build a working circuit using opto electronic components. I chose LDR because it's the only opto electronic component available near our place.

I just need to present it in class working properly.

 

[(*steve*)]

OK. As a demo it's probably suitable.

But I would advise you to make it up as soon as possible so you can iron out any quirks in its behaviour.

Also, what transistors are you intending to use?

I would recommend something like BC548's, but 2N2222's should also work.

I trust the LED is going to operate at low current? (i.e. you're not planning to use a 3W LED or something silly).

The circuit will be sensitive to the voltage it is operating from. If you set it up with a 9V battery and then try to operate it from 6V or 12V, you should expect the trigger levels to change (at the very least).

I also recommend that you understand the circuit before demonstrating it. One important thing to know is the voltage across R13 (how low can it go, how high can it go).

If you want to explain it back to me, I'll make sure you're not getting anything grossly wrong.

As a demonstration, you could also make a simpler circuit using an SCR. Perhaps one with an SCR and another with a transistor.

I guess it all depends on how happy you are with this and how much time you have.

 

[kamisochino]

Yeah I'll start doing it tomorrow.

I'll would probably go with 2N2222 because they are the most common here.

I would be using the common 5mm LED's probably the ones running around 20mA. I would also like to ask if it's ok to add a mini buzzer at the output just to add a little bit of effects.

I will also stick with a 9V battery.

I'm also planning to use a C106MG for the SCR with holding current around 20mA.

I'll report back as soon I have tested it. I still have a week before the deadline but If I won't make it by that time, I would just go with the ones I found on the net ( like this one in here http://circuiteasy.com/automatic-street-light/).

Thanks a lot again.