I translated a Russian article showing you how to build this useful device.

 

Mechanics know how difficult it is to find the source of background noise in a car. There are lots of sources: engine, transmission, exhaust system, wheels, body elements, air stream flowing around the car when driving, etc.

Here's a simple device that could assist in the identification and analysis of sources of noise of various nature, emerging only occasionally at a certain speed and load. This device allows to diagnose the condition of bearings, valves, injectors, brake pads and many other mechanisms and parts.

 

Fig.1

 

Schematic is presented in Fig. 1. The signal from the noise sensor BQ1 is fed to the input of amplifier DA1. Variable resistor R1 allows you to adjust the sensitivity of the device while listening to noises of variable intensity. The output signal is supplied to headphones. Gain - up to 26 dB. Current consumption in stand-by mode - not more than 9 mA.

 

Fig.2

 

The design and dimensions of the sensor are largely arbitrary (Fig. 2). It consists of three parts - the base, the sensitive piezoelement, and the flat magnet, allowing easy installation and removal of the sensor to/from the monitoring place.

The base is made of metal - steel, copper, brass or aluminum. As the sensor of vibration I used a piezoelectric buzzer, (such as CP-19, CP-W) found also in musical greeting cards. The sensitive element can be soldered to the base or epoxy glued. After soldering the flexible shielded cable (length is not critical), the piezo element must be protected from moisture with a sealant. The shape of the basis has to provide protection to sensitive element against casual blows.

On the other side of the base of sensor element, glue (or attach rigidly) a small flat magnet. It must be pretty strong to hold the sensor securely even on suspension parts when car is driving on rough roads. I used the magnet from a faulty hard disk from a PC.

Power the device from a 9V battery.

Any headphones with a resistance of about 8 Ohms are suitable. It is possible to use widespread stereo headphones with a resistance of 2x18 Ohms by having them connected in parallel at X2 socket. If the phones volume is not enough, it can be increased by connecting a 10 uF/16V capacitor between pins 1 and 8 of DA1 (positive to pin 1).

 

Fig.3

 

All parts of the device, except for the sensor, battery, variable resistor and connectors are mounted on a printed circuit board made of 1 mm fiberglass. The board is shown in Figure 3. After testing, the board should be covered with weatherproof paint.

The finished board is placed in a suitable sturdy box with a compartment for the battery and space to set the sensitivity adjustment, power switch and connectors. Power switch SA1 can be combined with the variable resistor R1.

The assembled device doesn't need any adjustments. Complete device is shown in Fig. 4.

 

Fig.4

 

Note:
You can build up to 4 noise sensors and add a 4-position commutator to input X1. This way you can switch between them while the car is moving to pinpoint the noise location.

you can paint the sensors in different colors and use same colors for jacks at the end of the wires.

then mark the sockets and commutator positions with same colors.

every sensor channel is now properly identified.

I was doing some repairs to a vintage record player and found Maplin a good source of components in the uk. If anyone's planning building one of these sensors I think they have most of the parts at www.maplin.co.uk.

Not sure they have the noise sensor shown at BQ1; is it some sort of miniature microphone?

Not sure they have the noise sensor shown at BQ1; is it some sort of miniature microphone?

not quite. it is a piezoelectric transducer.

it was widely used in cheap made in china land telephones as headphone.

it has some 3 cm in diameter (photos below)

see also here.

 

normally you apply a voltage to it and the metallic diaphragm attached to piezo element vibrates producing sound.

but if you apply vibration to piezo transducer, it produces a voltage at output wires.

that voltage is then amplified by up to 200 times by the circuit presented.

 

Edited 28 July, 201312 yr by dohnjoe

in fact Maplin sell it too.

 

http://www.maplin.co.uk/piezo-transducers-3202