Geiger Counter Kit

To build this kit, you should know how to solder. And it’ll be much easier if you’ve made other kits before. But even if this is your first kit, don’t despair! Just take your time and you’ll be fine.

If you’ve never soldered before, Andie Nordgren, Mitch Altman, and Jeff Keyzer of MightyOhm made a fun comic book that will teach you how. You can find it online, translated into many languages, at mightyohm.com/soldercomic.

We recommend soldering one resistor at a time unless you know what you’re doing!

The resistors can go in either way — they’re not polarized. Bend the leads of each resistor straight down and push them through the holes in the board, then bend them out at a 45° angle to keep the resistor from falling out.

Turn the board over, and solder each lead. Then, cut off the excess leads with your wire cutters.

Diodes are polarized, so they have to go in the right way. Make sure the stripe lines up with the black band shown on the board!

D1 is larger, and black with a white stripe.

D2 is smaller, and reddish-orange colored, with a black stripe.

Both are labeled with the part number, although it can be hard to read.

Make sure it sits flat on the board with the plastic lever sticking off the edge. I like to solder one of the end tabs, then reflow the solder and push it flat with my finger (don’t leave the soldering iron on it too long, or you will say “ouch!”)

Be sure to solder both tabs on the ends as well as the 3 leads.

Both of these capacitors are very small. C4 goes next to U2 and is marked 104. It’s the very tiny capacitor with narrowly spaced leads (2.5mm).

C3 is marked 102 and has wider spaced leads (5mm).

The variable resistor is easy to find — it’s blue with 3 leads.

First, solder Q1. It’s marked J3303. Make sure the flat spot of the transistor lines up with the markings on the PCB. You’ll need to bend the leads slightly to get them into the holes.

NOTE: Don’t push the transistors all the way down against the PCB — leave a few millimeters gap.

Solder Q2 and Q3, marked 3904. Make sure you put them in the right way, following the markings on the board!

If you want to log data or hack the kit, install the headers J5, J6, and J7.

J5 is the double-row header (2 rows × 3 pins).

J6 and J7 are single-row headers. Carefully break the supplied header strip into 1 strip of 3 and 1 strip of 6 pins, using pliers or wire cutters.

When soldering the headers, it’s a good idea to solder one pin first, then make sure the header is straight, and then solder the rest of the pins.

C2 is marked 221K and sits up above the PCB a little bit.

X1 is marked 8.00M or 8.0MC and has 3 leads.

Both of these can be installed in either direction — they are nonpolarized.

U1 and U2 pop into place and are easy to solder. Make sure the notch in the socket aligns with the notch in the silkscreened image on the PCB. You don’t need to clip the leads, they’re already very short.

Important: If you’ll mount your Geiger counter in the included acrylic case, use the pushbutton that comes with the case hardware (it has a long shaft) and solder it last, after all the other components!

The pushbutton can go in either way. No need to clip the leads on this either.

L1 is the black cylinder marked 103. It can go in either way.

LS1 is the large, short cylinder with a hole in the middle. It can go in either way.

C5 is the large black capacitor with a white stripe on one side — that side is negative and goes away from the + symbol. Make sure you install this one in the right direction, because it’s polarized. Push it all the way down against the PCB. You might have to wiggle the leads a little bit.

These are the small metal clips. Positions J1 and J2 are the furthest apart; use these for the SBM-20 Geiger tube included in the kit. To substitute shorter tubes, you can put the right-hand clip into different positions.

Solder one lead of the clip, make sure it’s straight, and then solder the other lead.

The LED is polarized, so be sure to put it in the right way. It will sit up a bit above the PCB.

The longer, positive lead goes into the hole marked +, and the flat spot on the side of the LED indicates the shorter, negative lead (just like the drawing on the PCB).

Solder one lead, check to make sure the LED is sitting straight, and then solder the other lead.

C1 can go in either way.

Make sure it’s straight, cut off the excess leads (watch your eyes!), and insert the small screw into the hole in the middle. Flip the board over and install the small nut. You’ll need a small screwdriver, but you can use your fingers to hold the nut.

You’ll need to bend the IC pins inward slightly to get them to fit into the sockets. Be careful! You don’t want to push too hard and wind up with a pin bent and sticking out of the socket — maybe into your finger!

Very important! Make sure the notch and/or small dot on each IC goes toward the notch in the socket (on the left, if you’re looking at the board with the text right-side-up). The ATtiny2313 has a large dot and a small dot on top — the small dot goes closest to the notch (pin 1).

The TLC555CP chip goes into the socket for U1.

The ATTINY2313V chip goes into the socket for U2.

Handle these parts carefully, as they’re static sensitive.

Important! First, use a pair of needlenose pliers to bend the tube clips apart slightly, so they aren’t so tight on the tube. Then carefully insert the tube. It is polarized, so make sure the + mark or CBM20 label is on the left side of the board, closest to the tube clip marked +.

CAUTION: Don’t push too hard on the Geiger tube, or drop it – it is very fragile. If the clips are too tight, stop and bend them outward slightly, then try again. Don’t force the tube into the clips or you will crush it.

WARNING: This kit is capable of generating high voltages (300V–600V). Don’t touch any part of the circuit (particularly the bottom of the PCB) while it is on! The current is low, so while the potential for injury is low, it may startle you and cause you to drop your kit (and break your Geiger tube!).

Make sure the switch is off (away from the ON position). Then insert two AAA batteries in the battery holder.

Use a small screwdriver to rotate the adjustment on VR1 all the way to the left (CCW) and turn on S1.

Listen for clicks and watch the LED. Slowly, rotate VR1 clockwise until you hear clicks. Once you start to hear clicks, rotate VR1 about 45° more and stop.

You’re done!

If you have a multimeter with a very high input impedance (1 gigohm), you can measure the voltage from ground to TP2 – it should be about 400V.

If you have an ordinary digital multimeter (DMM), you can get a 1 gigohm (1GΩ) resistor and put it in series with your DMM, then multiply the reading by the ratio [1,000 / (the internal resistance of your DMM in megohms)].

Now’s the time to install the tall pushbutton switch that comes with the case (instead of the short one from Step 8). Solder it at S2.

Remove the paper backing from the acrylic top and bottom plates. Use your fingernail, not anything metallic or sharp, because the acrylic scratches fairly easily.

Install the standoffs on the 4 corners of the PCB. The longer, round standoffs go on top of the board, and the shorter hex standoffs screw into them through the bottom. Finger-tight is OK, just make them snug.

Secure the bottom plate to the hex standoffs with a screw and lock washer at each corner. Tighten the screws until snug, but don’t over-tighten — if you do, you’ll crack the acrylic.

Turn the kit over and install the top plate with the remaining screws and lock washers. Again, don’t overtighten the screws.

You’re done!

Cleaning the case: Use a soft cloth (microfiber is best). Gently wipe the case clean, being careful not to scratch it. If necessary, you can remove the cover plates and wash them in soap and warm water. Never use alcohol or ammonia-based cleaners (such as Windex) as they will damage the acrylic and may cause it to crack.

Want to give your Geiger Counter Kit a little more sparkle? Replace the ordinary indicator that comes with the kit with something special: a superbright LED! Follow the directions at MightyOhm.

Got an iPhone or other iOS device? Here’s how to use the Geiger Counter Kit with the Geiger Bot app on your iOS device. Nice!