curcus boards electronics

Police Detective Electronic Circuits

This kit contains circuit diagrams, instructions and all the components to make:-

(Also there are small unprinted circuit boards and chip socket to get started with.)

All you will require is solder, soldering iron, something to cut and strip wire and a 9v battery also a breadboard is optional to get familiar with the electronics for the inexperienced.
Buying this kit of useful components is a great way to learn electronics and get started. This kit has a balanced range of components to introduce many aspects of electronics (see notes for insight).
To keep the price of the kit down some of the components are shared between circuits. The lie detector can be assembled on one board and the Police siren or ticking bomb on the other but the speaker will have to be exchanged between the two. Notice that the speaker is durable and high quality.

main components

  1. Instruction manual
  2. 2x  NE555P(timer IC’s)
  3. NPN Transistor (2N3904)
  4. PNP Transistor (2N3906)
  5. Loudspeaker
  1. 2x Electrolytic capacitors (10uF)
  2. 3x Ceramic capacitors (10nF, 2x 100nF)
  3. 8x Resistors (1K, 4.7K, 3x 10K, 47K, 68K, 82K)
  1. Heat Shrink tube
  2. Wire (1 meter)
  3. 18 pin chip mount on circuit board & circuit board
  4. PNP Transistor (2N3906)
  5. Switch

Warning: electrical components can be damaged if they are incorrectly wired, so I suggest if your circuit is not working when you think it should; disconnect the battery immediately which will reduce the chance of damage to components and battery.
The main causes of the circuits not working are gaps in soldering (dry joints) or unintentional connections (short circuits), incorrect wiring and plugged in components, then heat damage from the soldering iron or physical damage (such as jumping up and down on a circuit that refuses to work).
Things you need to do to complete project: Understand the right way round for components, cut & strip wire for connectors, speakers and jump wires. Assemble, trim, solder components and connect battery (I put my working circuit in an improvised box to stop it getting damaged).

Lie detector

Component list:

Loudspeaker, circuit board, switch, Capacitor (10nF), NPN Transistor (2N3904), PNP Transistor (2N3906), resistors (82k & 4.7K), wire.

lie dectctor
lie detector

The function of this lie detector circuit is based on the variation of the skin resistance that is used to vary the frequency of a tone oscillator. The contact rings can be two bars rings, about ¾ “. This detector circuit is basic unlike other detector circuits (professional devices) which electronically measures changes in respiratory rate and volume, heart rate and blood pressure plus sweat gland activity that indicate arousal of the autonomic (involuntary) nervous system, but you don’t have to tell your friends that. The probes can be used to test the moisture level and conductivity of 100s of interesting things, even test if a cake is cooked or not.

lie detector If you have never soldered anything before find someone who has to get you started, soldering can be tricky, leading to sour burnt finger and melt holes everywhere accompanied by blobs of escaped solder.
Try using some heat shrink tubing. This insulates the components wires from each other. Place a heated soldering iron close but not touching the tube and move along the length of the tube and it contracts. It’s a fiddly process at first avoiding burnt fingers. This circuit uses both a PNP and NPN transistor.

See notes to find out more about transistors. With the flat side of the transistor top and the connectors going down, from left to right you have the Collector (C), Base (B) and Emitter (E) which need to be wired in correctly. Exploring the best way to get an even connectivity on the probes will improve results.

Ticking Bomb

Component list:

Loudspeaker, 18 pin chip mount /circuit board, switch, electrolytic Capacitors (2 x 10uF), IC (555 timer), resistor (47K), wire.

lie dectctor
lie detector

This circuit simply makes an authentic sounding ticking bomb. With a bit of imagination and props the live wire between the battery and circuit could be combined with dummy wires of your choosing, only cutting one of these wires will stop the ticking and win you the game. I found it a great party game.

lie detector

The pin numbering system of the 555 timer starts and ends at the notch (from the top of the chip) going from 1 anticlockwise to 8. Evenly press the chip into the 18 pin chip mount on the circuit board and solder the components around it. See notes for more information.
The Electrolytic capacitors have to be connected the right way round with the negative (-) side indicated in lighter shade on component.

The obvious addition to the this circuit would be a and explosion sound when one of the dummy wires are cut, if you are experienced in electronics you could use the other 555 timer as a basis to make a simple explosion sound. Check the website for our solution at a later date.

Police Siren

Component list:

Loudspeaker, 18 pin chip mount/circuit board,  Capacitors (2 x 100nF), IC (2 x 555 timer), switch,  electrolytic Capacitors (2 x 10uF), wire, resistors (1k, 3 x 10k, 68k).

lie dectctor
lie detector

The Police Siren circuit uses two 555's to produce an up-down wailing sound. The first 555 is wired as a low-frequency oscillator to control the VOLTAGE CONTROL pin 5 of the second 555. The voltage shift on pin 5 causes the frequency of the second oscillator to rise and fall.  

lie detector

It is possible to build the police siren circuit on the board provided but beginners be aware it is not the easiest way to build the circuit. Using a good size breadboard will give you chance to spread out and organize the circuit better.
Congratulations if you have got this far and completed the police siren. If you weren’t at the start you will be well on the way to tackling bigger projects. Look at:

For project ideas and other well designed and intriguing kits.    


Notes on Components

The Resistor resistor

Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED. Resistors may be connected either way round. They are not damaged by heat when soldering.


The Capacitor
Capacitors store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.
This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.
Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

  • µ means 10-6 (millionth), so 1000000µF = 1F
  • n means 10-9 (thousand-millionth), so 1000nF = 1µF
  • p means 10-12 (million-millionth), so 1000pF = 1nF

Capacitor values can be very difficult to find because there are many types of capacitor with different labeling systems!