The old fashioned mechanical bicycle horns are slowly getting discarded now and the folks are more interested to replace them with the musical horns imitating well as phone ringtones. One such project is discussed in this article. The circuit is very easy to build as it incorporates just a couple of active parts and a few other passive parts. The circuit can be operated with 3 volts DC through two penlight AAA size.
Electronic hobbyists who also own a bicycle will love this project. The proposed idea will help you to get rid of your old mechanical bicycle horn with a brand new loud electronic horn. Its a homemade project - another aspect that will amuse the young folks. Lets learn the whole procedure here.
Circuit Description and Construction Clues
Referring to the figure we can see how simple it is to construct the proposed circuit as it utilizes very few electronic parts. The transistor T1 is an ordinary general purpose transistor, the well known 8050. An 8050 is more powerful than the usual BC547 types and is able to handle current up to 150 mA comfortably. The transistor also owns the property of having greater hFE levels than other similar types of transistor resulting in better amplification of the music, and yes it is there basically to amplify the music source.
The music source here is the incredible IC UM66 which has an embedded piece of music “written” inside it. It just needs a supply voltage of 3 V (not to exceed) to get going. The pin-outs are also pretty simple to understand. The left one is the negative, center one is the positive and the right leg is the output – simple isn’t that?
Once the relevant supply terminals of the UM66 are assigned to their posts, it starts “singing” right away through its output pin. However, this audio level is very low and needs to be amplified before feeding it to the step-up coil. This is done by T1 as explained above and the amplified signal is sent to the coil.
The coil used here actually acts as a step-up transformer and is primarily used for stepping up the amplified music fluctuations from the transistor T1. The coil just like any other transformer as a primary and a secondary sections, however the sections are not isolated, rather are wound as a single winding with the center tap appropriately pulled out at the relevant calculated step.
The primary and the secondary winding leads are identified by measuring the corresponding resistances using a multi-tester. The leads which show lower resistance is the primary winding, and the one which shows relatively higher value is the secondary winding.
Normally the primary section will indicate a value of around 22 ohms while the secondary shows a value of around 160 ohms. The common lead across the measurements is the center tap and goes to the positive supply.
The piezo plate which is responsible for the actual reproduction of the sound is connected across the secondary winding directly. The terminals of the piezo from the central white area and the outer metal rim, both the areas are solderable, however soldering the connection over the inner circle needs great care, make sure the solder tip is lifted as soon as the solder spot is made, otherwise the white ceramic coating will immediately get burnt reducing some efficiency of the device. Another aspect with the piezo element is its installation or the fixing method.
The fixing is done over a plastic dish or cap having some depth (around 5 mm) and an inner elevated step of about 1.5 mm in height and 1 mm in width, covering the inner bottom edge of the cap (see fig). The inner diameter of the cap is such that the piezo just brushes inside the cap and settles over the elevated step. And it’s exactly how the piezo is placed and stuck inside the cap (see figure).
The sticking can be done by some good quality synthetic rubber based glue (as used for sticking rubber and leathers). The opposite surface of the cap has a central hole of some calculated diameter (say around 7 mm) and it determines the loudness of the generated sound from the piezo element. Varying this diameter of the hole can drastically vary the amplification and sharpness of the music intensity.
Once the entire wiring of the circuit and piezo assembly id completed, the unit can be powered using two penlight cells, which gives the required 3 volts to the circuit. Amazingly even with such low power supply the music intensity can be found to be significantly loud and ear piercing.
However the supply must not be exceeded this value because the IC UM66 cannot tolerate anything above 3 volts. Of course the unit can be used with higher supply voltages, up to 12 volts only if the supply to the IC is checked and regulated to 3 Volts by a resistor and a zener network. With 12 volts supply the amplification becomes very high and in fact becomes very compatible with cars for using as musical reverse horns.
Parts List
All resistors are ¼ watt, CFR, 5 %, unless otherwise stated
R1, R2 = 1 K,
T1 = 8050,
Coil = As shown in the diagram,
COB = UM 66 IC or any other similar type.
Piezo = 27 mm, two terminal type, as shown in the diagram.
PCB = Veroboard or any general purpose PCB.
As the amplifier circuit also uses a number of frequencies to drive the loudspeaker, the bass frequencies will be reduced. Therefore need to be plugged the bass-boost control at fedback loop amplifier, this is done to overcome the decline in quality.
