Pocket Photodiode Geiger Counter

Build a DIY geiger counter that uses a PIN photodiode as a substitute for an expensive Geiger-Mueller tube. It detects alpha and beta radiation particles. The circuit is soldered onto a small protoboard and everything is placed in an aluminum enclosure. Copper tubing and a piece of aluminum foil is used to help filter out noise and RF interference.

Required tools and parts:
Soldering iron and solder Aluminum foil Electrical tape Drill Step drill bit Hand tools Jumper wiring 3 CR1620 coin-cell batteries, or any coin-cell batteries to add up to 9V 2 pieces of copper tubing available at Home Depot.

1 BPW34 PIN photodiode

1 LM358 Dual Op-amp

1 2N3904 NPN Transistor

1 2N7000 Transistor

2 100nF Capacitor

1 100uF Capacitor

1 10nF Capacitor

1 20nF Capacitor

1 610Mohm Resistor

2 1.5Mohm Resistor

1 56kohm Resistor

1 150kohm Resistor

2 1kohm Resistor

1 250kohm Potentiometer

1 Piezo Buzzer

1 Toggle Switch

2 Crimp pins

1 2-pin Male Header

1 2-Position Housing Connector

1 Potentiometer Knob

1 8-pin IC socket

1 Protoboard

1 Fliptop Box

These products are kitted at:

http://bit.ly/1p2Zmhp

Since there are quite a few components that need to fit onto this small protoboard. positioning of each component is important. The IC socket should be around the middle of the board since the pins need to be accessed as you solder. Make sure the notch of the socket is positioned correctly as you want the IC to be placed.

Follow the schematic paying close attention to the pinouts of the transistors and dual op-amp IC.

As you can see from the schematic, pins 1 and 6 are tied together. Soldering the two pins from underneath the board is the best way to have a clean connection and save space at the same time. This technique will be used throughout the build to keep the entire circuit compact.

Solder each resistor so that they only take up 2 solder holes. The connections are made underneath so make sure you do not short any of the components by soldering its own leads together.

Place 20nF Capacitor
This capacitor is non-polarized. It connects to ground, which I am using as the bus on the protoboard.

Place 10nF Capacitor:
This capacitor goes from pin 5 to pin 7. It is non-polarized.

The Light Shield section of the schematic is going to be placed in a copper tube to help block out any electromagnetic interference and light coming into the photodiode. Soldering the components in a tube-like shape ensures a snug fit into the copper tube. Start by placing the photodiode on top of the 2N7000 MOSFET and soldering the gate (pin 2) to the anode of the photodiode (without the notch).

1.5Mohm Resistor to Photodiode
The cathode of the photodiode (with a notch) is connected to the 1.5Mohm resistor.

56kohm Resistor
Solder one end of the 56kohm resistor to the drain of the 2N7000 (pin 1), and the other end to the 1.5Mohm resistor.

100nF Capacitor
Solder the 100nF capacitor to the cathode of the photodiode and leave the other end unconnected.

Add one more jumper wire to the drain of the 2N7000 (pin 1). This will connect to pin 3 of the dual op-amp IC. To make sure these solder connections aren't accidentally shorted, place electrical tape around each lead.

There will be 4 connections made from the Light Shield section to the protoboard: Two connections go to ground, one connection goes to pin 3 of the dual op-amp, one connection goes to the emitter of the 2N3904 transistor.

Add a jumper wire to connect the Light Shield section to pin 3 of the op-amp. Solder the 2N3904 transistor, connecting the emitter (pin 3) to the jumper wire between the 1.5Mohm and 56kohm resistors.

Solder the 100uF capacitor, 100nF capacitor and 150kohm resistor to the circuit around the 2N3904 transistor. Pay attention to the pinout of the 2N3904 and the polarity of the 100uF capacitor. There is a small "+" on one side of the capacitor. Connect the other side to ground.

These wires will make it easier to connect to the protoboard. Solder the 250kohm potentiometer and 1kohm resistor to the protoboard.

Note: The photo shows a 100kohm potentiometer, but was replaced with a 250kohm potentiometer.

Make sure the polarity of the buzzer and LED are correct. The buzzer has a "+" on one end of the sticker, and the longer lead of the LED is connected to the 9V supply. The coin-cell battery holder is marked with a "+" and "-" on top and bottom. Make the solder connections accordingly onto the protoboard and add jumper wires to the bottom of the board. Part of the battery holder was shaved off in order to fit into the enclosure. If you plan your layout better, you won't have to.

Get the copper tubes and tape them together with some electrical tape. It should be the approximately length of the Light Shield circuit and fit snugly.

Drill a 3/8" hole into the side of the aluminum enclosure where the Light Shield section of the circuit will fit through. Also drill a 1/4" hole on the long side of the enclosure for the toggle switch.

Drill a 3/16" hole and 1/4" hole into the lid of the aluminum enclosure for the LED and potentiometer, respectively. Position the boardinto the enclosure and mark the lid before drilling the holes.

Lay down some electrical tape on the enclosure to prevent any accidental short-circuit connections.

Connect two small pieces of jumper wire with the crimp pins and solder the other ends to the switch. Slide the crimp pins into the header until it snaps in for a secure fit. Completed toggle switch connection.

Solder the 2-pin header to the board for the switch connection.

Screw on the toggle switch and insert the socket into the 2-pin header.

Everything is now electrically connected. Insert the batteries and screw on the lid. I used three stacked CR1620 coin-cell batteries, but you should be able to use any coin-cell batteries as long as it adds up to 9V. Place the copper tubing over the Light Shield section and place tape around the tubing to keep it from moving around. This also keeps light from interfering with the photodiode. Screw the knob onto the potentiometer and you are ready to begin detecting radiation particles. Note: This should only be used for recreational and entertainment purposes. It is not safe to be around high sources of radiation and this device should not be used as a reliable source of detecting harmful radiation in the environment.