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This project detects a visible laser beam striking a TEPT5600 phototransistor and reports the hit with two LEDs. A BS170 MOSFET converts the sensor signal into a logic-level event on PICAXE pin C.3: the green ready LED turns off, the blue hit LED turns on, and the programmed delay returns the circuit to ready mode.
It is a practical indoor electronics exercise and a useful starting point for toy-target experiments. It is not a laser power meter, rangefinder, or dependable outdoor security alarm.
What the circuit does—and where it fits
The original design, published February 16, 2016, is intended mainly for low-power laser-pointer or toy-gun targets. It can also demonstrate a remote-control or beam-break concept, but its simple, adjustable DC threshold is vulnerable to bright or changing light. Treat security use as a proof of concept unless you add optical and signal processing improvements.
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- Good fit: controlled indoor target practice, educational demonstrations, and PICAXE learning.
- Possible with changes: remote sensors, scoreboards, sounders, servos, and event logging.
- Poor fit without redesign: unattended outdoor detection or any application where false alarms are unacceptable.
The original source is All About Circuits’ PICAXE laser detection project.
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Safety before applying power
- Never aim a laser at a person, vehicle, aircraft, or a reflective surface.
- Use the lowest practical visible power, keep the beam below eye level, and terminate it on a matte, non-reflective stop.
- Do not use an invisible infrared source casually; the absence of a visible blink response makes exposure harder to notice.
- If you use a laser-equipped firearm insert, follow all firearm-handling rules and keep the beam path controlled.
- The source does not specify wavelength, optical power, or laser class, so no particular pointer or insert should be assumed compatible or safe.
How the signal flows
Laser → Q1 phototransistor → VR1 sensitivity network → Q2 BS170 MOSFET → PICAXE C.3 → LED state change
Sensor and threshold
Q1 is a TEPT5600 phototransistor. VR1 provides adjustable sensitivity so the threshold can be set above the room’s ordinary light while retaining response to the beam. The PICAXE is not measuring calibrated optical intensity; it receives a thresholded logic event.
Verify Q1’s emitter and collector before powering the circuit. In the original illustrated assembly, the emitter is identified by the green wire and the collector by the red wire.
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MOSFET interface
Q2, the BS170 N-channel MOSFET, isolates the light-sensitive stage from the microcontroller input. When the sensor is illuminated, Q2 changes state and pulls PICAXE input C.3 low. The firmware interprets that low level as a hit.
Indicators and timing
The green LED indicates ready/waiting; the blue LED indicates a detected hit. VR2 adjusts the shoot-time or ready interval. The source notes that timing values are expressed in milliseconds and can be changed in the program.
Parts and sourcing
The following is the original bill of materials. The linked distributor references date from the 2016 publication; check present-day stock, package, pinout, and electrical specifications before substituting.
| Reference | Part | Qty. |
|---|---|---|
| J1 | 3.5 mm, three-conductor programming jack | 1 |
| C1 | 0.1 µF ceramic capacitor, 50 V | 1 |
| R1 | 22 kΩ, 0.25 W resistor | 1 |
| R2, R3 | 10 kΩ, 0.25 W resistors | 2 |
| R4, R5 | 330 Ω, 0.25 W resistors | 2 |
| LED1 | Blue T1¾ LED | 1 |
| LED2 | Green T1¾ LED | 1 |
| Q1 | TEPT5600 phototransistor | 1 |
| Q2 | BS170 N-channel MOSFET | 1 |
| VR1 | 100 kΩ potentiometer or trimmer | 1 |
| VR2 | 10 kΩ potentiometer or trimmer | 1 |
| U1 | PICAXE-08M2 | 1 |
You also need a solderless breadboard, hookup wire, mechanical sensor mounting, and a regulated, filtered 5 V DC supply. Original component references include the PICAXE-08M2, TEPT5600, BS170, 22 kΩ resistor, 10 kΩ resistors, 330 Ω resistors, 100 kΩ trimmer, 10 kΩ trimmer, blue LED, and green LED.
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Assembly and wiring checks
- Build the control circuit on a solderless breadboard using the published schematic as the wiring authority.
- Orient the PICAXE correctly and confirm its pin numbering.
- Wire the programming jack exactly as shown in the schematic; programming requires compatible PICAXE hardware and software.
- Install the 330 Ω current-limiting resistor in series with each LED and observe LED polarity.
- Place the 0.1 µF bypass capacitor close to the PICAXE supply pins.
- Connect Q1’s emitter and collector correctly, and verify the BS170 pinout rather than assuming a substitute has the same lead order.
- Use a regulated, filtered 5 V supply. Inspect breadboard rail breaks, ground continuity, and accidental shorts.
- Program and test the circuit before enclosing it.
Mounting Q1 and using a lens
Q1 can remain on the main breadboard or be mounted on a small carrier and connected remotely with two wires. Aim it directly along the expected beam path and shield it from stray room light.
The original build reports a sensor carrier about 42 mm × 42 mm and a Fresnel lens that increased the effective target area from roughly 5 mm to 28 mm. Those are results of that particular sensor, lens, spacing, and alignment—not universal specifications. A lens makes aiming easier but can also admit more background light and narrow or reshape the field of view.
Programming the PICAXE
The project source provides a downloadable archive named Reactive_LASER_Target.zip from the original project page. The article identifies timing values on lines 25, 31, 37, and 40, expressed in milliseconds. Because the archive itself is the authoritative source for the listing, download it and verify the code against your PICAXE editor rather than copying an unverified transcription.
At runtime, the firmware implements a simple state sequence:
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- Turn on the green ready indication.
- Wait for C.3 to be asserted by the sensor interface.
- Turn off green and turn on blue for the hit indication.
- Wait the programmed interval, then restore the ready state.
Changing delay constants changes the user-visible ready, hit, and reset timing. Keep line-number references in mind if you edit the original program; line numbers and values can move.
Calibration
- Power the circuit down.
- Turn VR1 fully counter-clockwise to minimize sensitivity.
- Turn VR2 fully clockwise to maximize the shoot-time setting.
- Power up and wait for the green LED.
- With green lit, turn VR1 clockwise until the blue LED activates.
- Turn VR1 slightly counter-clockwise so the blue LED is no longer triggered by the room light.
- Power down before repositioning or enclosing the assembly.
For normal operation, power up, wait for green, aim the laser at Q1, and confirm that green turns off and blue turns on. After the programmed delay, blue should turn off and green should return. If the detector is too sensitive, rotate VR1 counter-clockwise. If the ready period is too long, rotate VR2 counter-clockwise within the limits imposed by the program.
Test in stages
- Power and indicators: verify 5 V, ground, LED polarity, and the ready indication.
- Programming: download a minimal test or the project program through the correctly wired jack.
- Dark-state check: block Q1 and confirm that the circuit does not report a hit continuously.
- Short-range hit: illuminate Q1 with a safe, visible, low-power beam.
- Alignment: vary distance and angle to find the usable target area.
- Ambient-light test: repeat under the intended room lighting and after switching nearby lights.
- Remote-sensor test: only then try longer wiring, checking for noise and ground-reference problems.
Troubleshooting
| Symptom | Likely causes | Recovery |
|---|---|---|
| No LEDs illuminate | No 5 V, reversed PICAXE, missing ground, or LED orientation error | Check supply voltage, ground continuity, PICAXE orientation, and LED polarity. |
| PICAXE will not program | Incorrect 3.5 mm jack wiring, unsuitable serial adapter, missing programming ground, or software setup problem | Compare every jack connection with the schematic and use PICAXE-compatible programming hardware. |
| Blue LED always on | VR1 too sensitive, Q1 reversed, bright room light, or MOSFET wiring error | Recalibrate VR1, verify Q1 and Q2 pinouts, and shade the sensor. |
| Laser does not trigger | Beam misses the small sensor area, poor alignment, wrong Q1 wiring, or a weak/divergent source | Align at close range, inspect Q1 wiring, and consider a carefully positioned lens. |
| False triggers outdoors | Sunlight or changing illumination exceeds the simple DC threshold | Use shielding, wavelength filtering, modulation, AC coupling, or frequency-selective detection. |
| Hit never resets | Edited timing code, C.3 held asserted, or power/reset fault | Restore the original program, monitor the sensor state, and check power and reset wiring. |
| Works locally but not remotely | Long wires picking up noise or losing a stable reference | Use short twisted or shielded wiring, local decoupling, and a solid common ground. |
Known limitation: ambient light
VR1 can reduce triggering from ordinary indoor illumination, but it does not make the detector immune to sunlight, reflections, or rapidly changing backgrounds. A discussion of this project identifies ambient light as its principal weakness and proposes modulating the laser and detecting that frequency instead: Laser Pointer Forums discussion.
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Ways to improve the detector
Modulate the beam
Drive the laser with a known carrier and detect that carrier at Q1. AC coupling and frequency-selective processing reject steady illumination more effectively than a DC threshold.
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A filter matched to the laser wavelength can reduce unwanted light, but the original project gives no wavelength or power. Select a filter only after those values are known.
Add hysteresis
A comparator or Schmitt-trigger stage can prevent chatter when the sensor voltage sits near its threshold.
Use a photodiode front end
A photodiode with a transimpedance amplifier or comparator offers more controllable bandwidth and sensitivity than an unconditioned phototransistor.
Strengthen outputs
The PICAXE can command sounders, servos, motors, solenoids, scoreboards, or logging electronics, but higher-current loads need suitable driver transistors or MOSFETs and their own protection.
When to choose another architecture
A dedicated modulated infrared receiver is generally better in bright environments, although it changes the optical and electronic design. A camera can recognize a larger spot or pattern but adds software, latency, and optical calibration. A PIC16F1516 password-controlled tripwire is another alarm-oriented approach: Maker Pro laser tripwire project.
Keep the PICAXE design when simplicity, through-hole assembly, and learning are more important than environmental immunity. Redesign it when false positives are unacceptable, the beam must be supervised unattended, the sensor is far from the controller, or you need multiple encoded targets.
Buying and substitution guidance
No current official pricing or stock listing was published, so the 2016 distributor list is not an August 2026 quote. Start with PICAXE for the microcontroller and programming interface, Digi-Key for passives and optoelectronics, Jameco for hobby quantities, Edmund Optics for optical components, and Adafruit for maker-oriented breadboards and regulated supplies. For every substitute, check voltage ratings, LED current, transistor/MOSFET pinout, MOSFET threshold behavior, phototransistor spectral response, and mechanical fit.
Quick Recap
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