Parts List for 556 version
R1 = 330KC1 = 4.7uF/16volt, electrolytic R2 = 390KC2 = 2.2uF/16volt, electrolyticR3,R6 = 68K C3 = 100nF, ceramicR4,R5 = 33K C4 = 100uF/16volt, electrolytic S1 = on/off switch C5 = 47nF, ceramic Re = Reed Relay (glass tube) LS = 8 Ohm, 0.25-2watt. Others may work too.IC1 = NE556 (or use two 555’s) (I used a 2"/2W type)D1,D2 = 1N4001
With this circuit you can electronically simulate the noise of a diesel-train horn.The sound is triggered automatically as the train reaches a desired place on the track so you can produce the sound as the train approaches stations, level crossings, etc.
The circuit can be built simply by using one 556 or two 555 timers.The rest of the parts are readily available and not critical.The output power from the two-Tone train horn is adequate to drive a miniature loudspeaker directly so all that is required is a single 9V alkaline battery or a 9VDC wall-adaptor.
How it Works:
For the explanation, we use the second schematic diagram with the two 555’s.The sound generating part of this circuit is formed by IC2, a 555 connected as an astable multivibrator.The frequency of the sound varies with the overall charge and discharge times of capacitor C4.So, when the junction of D1 and 2 is high, the frequency of oscillation is set by the current through R4 plus that through R5, charging the capacitor.Similarly, when the junction of D1, 2 is low the frequency is set by the current through R5, (no current can flow through R4).When the charge current is less, therefore, the frequency is lower.
Integrated Circuit IC1, another 555, forms a monostable multivibrator with an ’on’ time of about 2 seconds.The multivibrator output (pin 3) is connected directly to the junction of D1/D2, and hence when the monostable is ’on’ the frequency is higher than when it is ’off’.
A reed switch under the track is operated by a magnet attached to the train as it passes and this triggers IC1.
Click here-==> to listen.
Parts List for 555 version
R1 = 330KC1 = 100nF, ceramic R2 = 390KC2 = 2.2uF/16volt, electrolyticR3,R5 = 68K C3 = 4.7uF/16volt, electrolyticR4,R6 = 33K C4 = 47nF, ceramic S1 = on/off switch C5 = 100uF/16volt, electrolytic Re = Reed Relay (glass tube) LS = 8 Ohm, 0.25-2watt. Others may work too.IC1,IC2 = NE5552 Watts or more is required for betterD1,D2 = 1N4001 sound. (prototype used a 3"(76mm), 2W type)
The 555 version is available as a KIT: [Train Horn].
Veroboard construction (perf-board) on a standard sized 10 strip by 24 hole board means that this project is quite easy to build.
The usual procedures and precautions should be followed but you will find that very few of the components are critical.In fact you may like to experiment with differentvalues to get other sounds.
Make your track breaks first with either the purpose-made tool or simply a hand-held 1/8″ drill bit.Next, insert all links and IC sockets, followed by resistors, capacitors and finally semiconductors.
Figure 2 shows connection details of the project along with overlay and underside-board views and you should follow it carefully.
Each timer produces a high and a low tone. If you don’t want to fiddle with the reed relay that is fine too; use a momentary ’on’ pushbutton switch insteed.The reed relay may required a strong magnet (experiment with it). Make sure the reed relay is positioned in such a way that the magnet is able to close the contacts, and the reed relay contacts are facing upwards.With the correct loudspeaker (I used an oval 2-watt, 8 ohm type which I salvaged out of an old computer sound system), the sound is very realistic!The larger loudspeakers can get very low audio.