I have created a simple light sensing circuit using 2 voltage dividers and a comparator. Basically, high resistance and the output from comparator is high.
I need to explore other options due to the fact the comparator needs more power than I have available. I would like to make this circuit with 1.5 volts and I've found a couple LED flash circuits that will allow me to handle that end w/ 1.5v
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but the comparator.... I dunno.
I thought I could use a voltage divider with a transistor, but when I sat down to actually make this circuit, I realized I couldn't figure it out.
I've researched op-amps as a comparator but that doesn't seem like a good fit either.
I would be interested in any ideas that you guys might have.
Sorry to post so many questions here, I've been really active on this project lately and I'm learning a lot here so it's hard to not come back ;)
You can use a simple two-transistor oscillator to pump up the voltage sufficiently to light up a white LED from a 1.5V source. Using another NPN transistor to kill off oscillations. Here is a try:
NPN = 2n4401 (created by AACircuit v1.28.5 beta 02/06/05
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Your variable resistor may not be right for this. As the resistance increases above about 470k, The thing will start oscillating, causing the LED to fire up. If you want it to go when there is light, you'll need to use a slightly different scheme. As it gets lighter, your LDR probably has less resistance. Thus, replace 'variable resistor' with a resistor that approximately equals the resistance of the LDR at the point you want it to turn on at. Then, put the LDR from the base of the leftmost transistor to ground. When it gets lighter, it'll turn of the leftmost transistor, allowing the oscillator to go.
Thanks Bob! I will try this out tonight, looks interesting. The only component I don't recognize is the one in parallel to the LED that says 100u, what is that?
Hi, Steve. Sorry about the delay. Let's recap the bidding. You've got a sensor that exhibits a variable resistance between 2K ohms and
64K ohms. When the sensor resistance is greater than 4K, you want an LED to flash about once a second. The catch here is that you've got a fresh Duracell AAA battery, and need to be able to have this thing flash for 45 days with that battery as its only power source.
We talked before about your power requirements, and how a standard oscillator won't do the job because of excess current which will eat up the battery. The National Semiconductor LM3909 was made exactly for flashing a standard LED off a single 1.5V cell with very low power drain. Unfortunately, they're not making it anymore. However, there are some parts still available if you look or scrounge, and there's always the possibility of building one out of discrete components, as linked in an earlier post.
National published an app note on the LM3909, called AN-154: "1.3V IC Flasher, Oscillator, Trigger or Alarm"
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On the second and third pages of the app note, it describes the circuit operation for the basic 1.5V flasher circuit shown in Fig. 2. From looking at their description and examining the feedback paths, it seems that the flasher will not oscillate if only we can keep pin 2 (the + end of the 300uF cap) from exceeding 1V. That shouldn't be too hard, if we can find a comparator to do the rest of the job. Here's the circuit (view in fixed font or M$ Notepad):
I'm speechless, that is the most awesome experience I've had on the web w/ maps.google.com following in a close second! :)
Thanks a million, Chris, I will go off this weekend and get everything I need to prototype this guy.
I'm thinking I will make the LM3909 w/ discreet parts on a separate breadboard, then make the sensing on another. That way I can get each one working separatly and then tie them together for the grand finale!
Thanks again, I really appreciate the time you spent on this.
I just noticed this schematic when looking for other information:
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It looks as though it may be interesting to your application, if I understand it to be a "blinking LED" that also "senses light" and works off of a 1.5V AAA battery.
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