Frustration

Making something is good and relatively easy, when it doesn't work and you try to find out why is when the real learning begins. Apart from those who are gifted and can learn with no effort I have no doubt the people giving you advice here have gone through the same experience. Just stay calm and double check every thing. Best of luck.

If you look at the schematic and have some idea in your mind on how it is supposed to work... you take your multi-tester and measure a few things like the junction of the resistor and LDR. If it is greater than .6 volts the transistor (assuming you used a bipolar junction transistor) should switch on.

Or are you using the 555 schematic? (The trigger for a 555 has to be less than 1/3 the supply voltage) 12V supply the trigger has to dip lower than 4V...

Use logic and think about what is supposed to be going on then use the meter to see what is really going on.

How specifically do I measure the junction of the resistor and the LDR?

I am using the 555 chip.

Andy

I asked how to specifically do this, but did not get a reply.

got a voltmeter to test pin 3&6? when pin 6 is >6v then pin 3 = 0v when pin 6 is

Are you know using the 555 circuit. Mikek

I do not understand your question.

Do you know of any circuit that will detect someone crossing a light path?

It does not necessarily have to use a laser.

Andy

Hmm, Well a common pitfall when starting in electronics is to copy all of some circuit. Plug it in and find it doesn't work. A better approach would be to build up one little part of it.. make sure you know how that little part is supposed to work, and confirm it's working as expected. (Or not and go figure out why.) Then add the next little piece.

Do you have an oscilloscope?

I'm not glued to the PC this morning...

Measurements are assumed to be to ground or voltage minus unless otherwise stated

I don't have the schematic in front of me but I think pin one is ground on the 555?

He is working on a pretty basic circuit. 555 monostable, with a LDR on the trigger and buzzer on the output.

OK I found the schematic again... and now I understand he's using the

555 as a simple latch to keep the buzzer on indefinitely

When there's light on the LDR Pin 6 should be low and pin 3 should be high. (presumably you toggled the toggle switch to set those conditions)

When light is interrupted Pin 3 goes low and turns on the buzzer until you flip the toggle switch to turn it off again

Do you have the toggle switch wired as shown? What voltage on pin 6 with light on the LDR, and what voltage on pin 6 with the light interrupted (blocked)? That voltage has the change significantly between light and dark.

When the voltage on pin 6 is greater than the voltage on pin 5, pin

3 should be high and the buzzer OFF.

When the voltage on pin 6 dips below 2/3d of your power supply voltage pin three should go low and stay low, and the buzzer should be ON until the toggle switch is toggled.

Electronics is frustrating if you put together things that you don't understand, and they don't work. Why not just buy stuff all done?

If you do want to learn about electronics, start with very simple circuits that you invent yourself, and do the math, and make it work.

Start with a battery and some resistors a DVM. Do the simple math all along the way.

After that makes sense, add one big capacitor. Then an LED or two. Don't move on until you understand it.

An introductory EE course at a community college would be hugely helpful.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Here is the schematic

He's using the 555 as a simple latch to turn on the buzzer until the toggle switch resets it.

Get a CdS photoresistor and a battery and a few resistors and a flashlight and a DVM and experiment.

Start by connecting just the photoresistor to the DVM and measure ohms.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I'm surprised you have already forgot about the first transistor circuit you posted and then my response with a google search with 25 more transistor circuits. After I posted you answered the question to someone else, yes you are using the 555 circuit. I'm disappointed, you had a chance to learn how to bias a transistor on or off and then see how the changing resistance of your LDR would cause the switch on and off. You should find it interesting to bias a transistor to have the buzzer quiet, change the value of one resistor and make the buzzer buzz. Then put the original value back in with the LDR in parallel and see light make the buzzer go on and off. You were to quick to abandon the original circuit.

Mikek

OK, I haven't used a 555 in decades, and have mostly forgotten how it works.

Maybe he could try a comparator and a S/R flip flop.. Or just a comparator to LED + resistor.

George H.

I have tested the individual parts of circuit that I familiar with. I do not have a scope.

I had need of a break-beam photo detector in the 60's to keep my motorcycle from being stolen. I used a potentiometer, photocell, and SCR for the latch to keep a doorbell ringing. (the inductive kick and contacts opening meant it wouldn't latch until I added an electrolytic cap across the bell) We didn't have integrated circuits.

The OP can be comforted by the fact that expecting a circuit to work first time after building it is unduly optimistic. Even simple circuits it's easy to overlook something, no matter how experienced you become. IME (very considerable over 50+ years) the majority of circuits do NOT work first time and so need time spend re-checking everything. You just get quicker and quicker at the checking and testing procedures!

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