First try on laser alarm

[fixit7]

I put this together on my breadboard. The author said that R1 was probably not needed, so I did not use it.

Laser Tripwire Alarm

No security system is complete without lasers. Learn to build a laser tripwire alarm from a laser pointer, a couple of mirrors, and a few electrical parts.

makezine.com

I do not have a potentiometer, but tried a 110 and 530 ohn resistor for R2.

With no power to the circuit, laser shining on the LDR, the reading was around 195 ohms.

I shined laser at the LDR and used the switch in both positions.

Specs for LDR


Features:

• Maximum voltage rating @ 25°C: 350VDC
• Maximum allowable power dissipation @ 25°C: 400mW
• Resistance (Dark): 1MΩ
• Resistance (Light): 12kΩ
• Lead length: 1.44"
• Operating temperature: -30°C to +70°C
• Able to withstand soldering at 230°C for 3 seconds

Piezo buzzer did not sound.

What can I try next?

 

[Dr_Doggy]

see instrucitons:
The value of R2 should be approximately the same as the resistance of the photoresistor when you are shining the laser pointer directly at the light sensitive face.

that brings r2 closer to "12K ohm" or "12000 ohm"

 

[fixit7]

But I got around 195 ohms?

How did you come up with 12K?

What specifically do these terms mean?

Resistance (Dark): 1MΩ
Resistance (Light): 12kΩ

 

[Mickster]

What specifically do these terms mean?

Resistance (Dark): 1MΩ
Resistance (Light): 12kΩ

The person putting the circuit together took readings from the LDR.
The resistance of that LDR when no light was shining on it was 1 Meg-Ohm.
The resistance of that LDR when the laser was shining on it was 12 K-Ohm.

 

[fixit7]

The values were from the spec sheet provided by Jameco where I bought the LDR.

 

[fixit7]

see instrucitons:
The value of R2 should be approximately the same as the resistance of the photoresistor when you are shining the laser pointer directly at the light sensitive face.

that brings r2 closer to "12K ohm" or "12000 ohm"

I will order a 15K pot.

Which type, linear taper or log taper?

 

[Dr_Doggy]

id even go with 30k,
but its a good idea to check the resistance of the dark too, and the rooms natural lighting, R2 is basically your sensitivity and dependent on the ldr.
a pot is nice for adjusting the sensitivity but for just basic testing a few resistors will do fine ... your main problem is r2 is too low in value

also holding the laser closer will drop the resistance lower .... is that how its going to be in the field?

 

[Dr_Doggy]

to test the buzzer hook it up to the battery direct,
to test the switch put it between the battery and buzzer

got a voltmeter to test pin 3&6?
when pin 6 is >6v then pin 3 = 0v
when pin 6 is <6v then pin 3 = 9v

 

[fixit7]

I put in(2) 6.8K ohms to get about 13.4K, but no change.

I noticed that when I check the ohms with the resistors in a non-powered circuit, I got no reading on any scale ?

I plan on shooting the laser across my front door to the detector. Patio is a shaded area.

The plan is for the interruption the beam to trigger the buzzer.

Eventually I will hit on the right value.

I think Edison tried around 2000 materials before he got a working filament.

 

[Dr_Doggy]

power up circuit and measure dc volts

 

[fixit7]

9.39 V at the power rail.
On meg ohm scale I get readings of my skin resistance.

 

[fixit7]

to test the buzzer hook it up to the battery direct,
to test the switch put it between the battery and buzzer

got a voltmeter to test pin 3&6?
when pin 6 is >6v then pin 3 = 0v
when pin 6 is <6v then pin 3 = 9v

Buzzer and switch work.

Where do I put the leads for pins 3 and 6?

Like where does the + and - specifically go to?

 

[fixit7]

"The value of R2 should be approximately the same as the resistance of the photoresistor when you are shining the laser pointer directly at the light sensitive face."

Do I measure that with a live circuit?

 

[Pommie]

I'm at school atm, so can someone do a simple 1 transistor solution? No need for a 555.

Mike.

 

[fixit7]

I may order this unless someone can tell what the components are.

**broken link removed**

It may be worth another $3.78 to get a working laser alarm.

 

[fixit7]

Buzzer and switch work.

Where do I put the leads for pins 3 and 6?

Like where does the + and - specifically go to?

I need some help with this please.

Thanks.

 

[KeepItSimpleStupid]

As Dr_Doggy said, look at pin 6 relative to ground. 2/3 Vcc is the magic number. @ 9V, this is 6V.

With light on the photocell, pin 6 relative to ground has to be below 2/3 Vcc. The photocell and R2 divide the battery voltage.
Instead of looking at formulas, let's look at basics . The current through R3 and R2 is I=Vcc/(R2+R3). The voltage cross R2 is I*R2 and the voltage across R3 is I*R3.

The part acts as a comparitor, latch and driver in this application. Datasheet: https://www.ti.com/lit/ds/symlink/lm555.pdf

Magic numbers are 1/3 and 2/3 Vcc for this part.

Piezo buzzers can be troublesome. You could have bought a piezo transducer or speaker, They wont work. There is a polarity for buzzers. make sure the buzzer works when directly connected to the battery.

PS: Relative to ground means black connected to negative. We will also assume the meter can display polarity for future reference. Ground of the (-) supply is USUSLLY the referenec in a circuit with a single supply (9V battery). Most voltages are measured with respect to a reference. Occasionally, you lokk at voltages across things.

You mentioned that you got No resistance across resistors. There is a lot of problems with no. Use low or ~zero, but not NO. "No resistance" can easily mean infinate. Modern meters are such that they don't turn on semiconductor junctions, but they can.

Make sure your meter reads close to zero when the leads are shorted. Leads have resistance, Might be as high as 0.5 ohms.

Watch your probes. Some that are really sharp actually have an insulator along most of it's length.

generally you have COM, (Volts/ohms), and a mA Jack. Since measuring current is almost like putting a direct short (low value resistor) across what your measuring, the current scale is protected by a fuse in a good meter. You also use different connections,

Ohms and volts use the same terminals, but measuring ohms in a powered circuit can destroy the meter. Capacitors have charge, so discharging them is a good idea.

Probe leads break or degrade when their muti-stranded wires break.

Probe leads have insulation ratings. When you have a chance loot at the CAT ratings of meters.

This stuff applies to most DVM's. High voltage, High frequency and extremely low currents like in nA, pA or fA ranges require much different rules.

Any meter disturbs the measurement. Most (not all) meters are designed for a 10 M (10,000,000 ohms) ohm input impeadance. Thus measuring anything is adding a 10 M resistor, thus measuring the voltage across the 10M cell will disturb it. It makes it now look like 5 M.

The formula for resistors in parallel is 1/Rt=1/r1+1/r2+....1/Rn

12k in parallel with 10M is is close to 11859. The percent error is close to 1% if measuring voltages. Tolerances already exist in the part values and with temperature primarily.

Measuring current can also add a resistor in series.

This blurb is not all inclusive.

Furthermore, you may have to out a tube around the photocell to take out ambient light.

This circuit is not really a good application. It will be dependent on stray light. Remote controls modulate their output, so the IR from the sun or lamps don't confuse it. A beam break detector over large distances for an IR LED should be modulated so ambient light is rejected.

So, the voltage across R2 is easier to measure. Compute the voltage across R3 as battery-R2

 

[fixit7]

Ok.

Meter reads zero on all ranges except 200 where it reads 1.2

With laser on photoresistor,

-3.0 for pin 6
-3.6 for pin 3

You say my speaker won't work and the circuit is not good.

Buzzer makes a beeping sound when connected directly to power supply.

Well, what circuit would work and it does not necessarily have to use a laser?

Someone crossing the light path and causing a shadow would work.

Or a motion detector would be good too.

 

[KeepItSimpleStupid]

With laser on photoresistor,

-3.0 for pin 6
-3.6 for pin 3

the voltages have to be + if they are respect to ground. Even if it's 9-3.6, it's > 6V aprox. We do nned to know what the 9V battery is.

best if we had (relative to ground) black lead plugged into COM on DVM and connected to (-) of battery.
1. Battery voltage (red lead)
2. pin 6, laser off (red lead)
3 pin 6 laser on (red lead)

R2 has to be increased probably.

Putting a jumper across the photoresistor briefly should turn the buzzer on. Laser not required.

 

[fixit7]

Strange results. I am using a power supply not a battery.
1. 9.42
2. 0
3. 0