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  • Author Author: tariq.ahmad
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DIY Laser Trip Wire Alarm -- The Learning Circuit 84

 

 

Now that we know how laser diodes work, it’s time to use one in a project! Karen uses a laser diode and an LDR sensor to create a “trip wire”. When the “wire” is “tripped”, meaning the laser beam is interrupted, lights will flash and an alarm will sound! To create the alarm siren, Karen couples together two 555 timer circuits. The entire project is triggered using a third 555 timer circuit used in bistable mode. And a relay circuit is used to control the motor and LED of the flashing alarm light.

 

Bill of Material:

 

Product NameManufacturerQuantityBuy KitBuy Kit
LM555CN - Timer, Oscillator & Pulse Generator IC, TTL Compatible, Astable, Monostable, 4.5 V to 16 V, DIP-8On Semiconductor3Buy NowBuy Now
LM317T -1.2V to 37V, 1.5A Adjustable Positive Voltage Regulator in TO-220-3 PackageSTM Electronics1Buy NowBuy Now
G5V-1-DC5 -Signal Relay, 5 VDC, SPDT, 1 A, G5V-1 Series, Through Hole, Non LatchingOMRON ELECTRONIC COMPONENTS1Buy NowBuy Now
Ceramic Disc Capacitor, 0.01 µF, 100 V, ± 20%, 5 mmVishay3Buy NowBuy Now
Electrolytic Capacitor, Miniature, 10 µF, 25 V, 515D Series, ± 20%, Radial Leaded, 5 mmVishay3Buy NowBuy Now
Through Hole Resistor, 10 ohm, MF25 Series, 250 mW, ± 1%, Axial Leaded, 250 VMulticomp1Buy NowBuy Now
Through Hole Resistor, 100 ohm, 250 mW, ± 1%, Axial Leaded, 250 VMulticomp1Buy NowBuy Now
Through Hole Resistor, 1 kohm, 500 mW, ± 1%TT ELECTRONICS / WELWYN1Buy NowBuy Now
Through Hole Resistor, 10 kohm, 500 mW, ± 5%, Axial Leaded, 350 VMULTICOMP4Buy NowBuy Now
Through Hole Resistor, 68 kohm, MF25 Series, 250 mW, ± 1%, Axial Leaded, 250 VMulticomp1Buy NowBuy Now
1N4007 -Standard Recovery Diode, 1 kV, 1 A, Single, 1.5 V, 30 AMULTICOMP PRO2Buy NowBuy Now
Trimpot, Single Turn, Cermet, Side Adjust, 220 ohm, Through Hole, 1 TurnsBourns1Buy NowBuy Now
Trimpot, Single Turn, Cermet, Top Adjust, 10 kohm, Through Hole, 1 TurnsBourns1Buy NowBuy Now
Laser diode - 5mW, 650nm, Red - ModuleAdafruit1Buy NowBuy Now
2.5 Inch 5.0W 4 Ohm Full Range Speaker, 66mmVisaton1Buy NowBuy Now
ADVANCED PHOTONIX LDR, Photoresistor, 1 Mohm, 250 mW, 320 VADVANCED PHOTONIX1Buy NowBuy Now
Rocker Switch, SPDT, On-Off-On, PanelZF ELECTRONICS1Buy NowBuy Now
Pushbutton Switch, SPST, Off-(On), Round, RedNTE ELECTRONICS1Buy NowBuy Now
Prototype Board, Phenolic, 30 x 25 grid, 1.6 mm, 94 mm, 71 mmMulticomp1Buy NowBuy Now
BATTERY HOLDER 4 X AA, 6V, w/ switch, wire leadsOSEPP1Buy NowBuy Now

 

Additional Parts:

 

Product Name

Rotating LED Beacon Light

Case materials

Extra Wire

(4) AA Batteries

Parents

  • Hi Karen,

     

    Thank you for that project. It is very instructive.  We all know now how automatic doors function.

     

    Your choice to connect pin 7 of the first 555 timer to VCC is being criticised on YouTube. It is said that this choice poses a short circuit hazard. If that is true than it is serious as short circuits are the main sources of fires. You have to address that.

     

    If I understood your circuit well than the first timer is actually a switch, turned on by the breaking of the laser beam. Wouldn’t a transistor be sufficient for that purpose ? As the beam reaches the LDR, the resistance accross the LDR is low and so is the voltage. But as the beam is broken, the resistance gets much bigger and so does the voltage. Isn’t that called biasing ? Aren't you abusing of 555 timers ? Be aware of that, it’s just like sugar, you need it but you must not abuse of it.image

     

    I don’t understand why all these guys on YouTube are fans of 555 timers. I must be missing something here. These chips are perfect for PWM as the so called duty cycle can be tuned. But, if you just need a rectangular wave with a duty cycle close to 50%, than a Schmitt-Trigger inverter is sufficient. Actually all these chips rely on an RC timing network to set the frequency of the wave but the latter one is easier to implement. This is my preferred oscillator, it’s even faster to implement than an operational amplifier based Schmitt-Trigger inverter .

     

    In the following video, this circuit is explained by a highly experimented professional:

     

    https://m.youtube.com/watch?v=NuXitMK3HSA

     

    Thanks again Karen.

Comment

  • Hi Karen,

     

    Thank you for that project. It is very instructive.  We all know now how automatic doors function.

     

    Your choice to connect pin 7 of the first 555 timer to VCC is being criticised on YouTube. It is said that this choice poses a short circuit hazard. If that is true than it is serious as short circuits are the main sources of fires. You have to address that.

     

    If I understood your circuit well than the first timer is actually a switch, turned on by the breaking of the laser beam. Wouldn’t a transistor be sufficient for that purpose ? As the beam reaches the LDR, the resistance accross the LDR is low and so is the voltage. But as the beam is broken, the resistance gets much bigger and so does the voltage. Isn’t that called biasing ? Aren't you abusing of 555 timers ? Be aware of that, it’s just like sugar, you need it but you must not abuse of it.image

     

    I don’t understand why all these guys on YouTube are fans of 555 timers. I must be missing something here. These chips are perfect for PWM as the so called duty cycle can be tuned. But, if you just need a rectangular wave with a duty cycle close to 50%, than a Schmitt-Trigger inverter is sufficient. Actually all these chips rely on an RC timing network to set the frequency of the wave but the latter one is easier to implement. This is my preferred oscillator, it’s even faster to implement than an operational amplifier based Schmitt-Trigger inverter .

     

    In the following video, this circuit is explained by a highly experimented professional:

     

    https://m.youtube.com/watch?v=NuXitMK3HSA

     

    Thanks again Karen.

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