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For Sound-Off competitions, neon underbody car lighting is a fantastic visual effect. Now you can make it pulse on and off with the beat of the music. (Photo: Julian Edgar). Neon tube modulator for cars & light systems This little circuit will modulate the 12V neon tubes which are now available for lighting systems in cars. It connects to the subwoofer signal from the car sound system and the neon light is modulated by the bass signal. By RICK WALTERS These days it is impossible to miss the evidence that some cars are fitted with very fancy and expensive sound systems. Often, the sound systems are worth more than the cars and owners go to great lengths when competing in “Sound-Off” competitions. One of the more unusual ways to dress up a car is to use neon tubes to light up the under body, the cabin and the boot space of the car. The neon tubes we are talking about are 24  Silicon Chip 955mm long overall and are housed in a tough plastic pipe for protec­tion. At one end of the pipe is a plastic case housing a high voltage inverter running from 12V DC. These tubes are available from Jaycar Electronics at $49.95 (Cat. ST-3126). Connection is simple: you just connect the tube up to 12V DC and it runs. Well, pretty neon lights are OK but they’re a bit static aren’t they? We thought to ourselves, “Why not make them pulse in time to the bass beat of the music?” That should give the gawkers at Sound-Off shows something to look at! The solution is a small PC board which will drive one or two of these neon tubes. There is an onboard trimpot to set the sensitivity and that’s about it; set and forget. Circuit description The circuit of the Neon tube modulator is shown in Fig.1. The subwoofer audio signal from the car’s sound system is applied to the input level trimpot VR1. From there, the signal is coupled via a 10µF capacitor to the base of transistor Q1. Q1 is lightly biassed on by the 330kΩ resistor and this means that its collector voltage is normally close to 0V. Provid­ed that the audio signal is more than about 1V peak-to-peak, Fig.1: transistor Q1 is fed with the subwoofer signal and acts to trigger the 555 timer in 15ms bursts to extinguish the neon tube. PARTS LIST 1 PC board, code 05105971, 60 x 28mm 1 8-pin IC socket (optional) 5 PC stakes (optional) 1 10kΩ horizontal mount trimpot (VR1) it will be sufficient to turn transistor Q1 on and off. The resulting signal at the collector of Q1 will be a poor square wave with a rounded rising edge, due to the 0.1µF capaci­tor across the 6.8kΩ collector resistor, but with a much faster falling edge, as shown in the waveform of Fig.2. The capacitor rolls off any higher frequencies that may be present since we’re only interested in sub­woofer frequencies; ie, 100Hz and below. Q1 is used to control IC1, a good old reliable 555 timer wired as a triggered mono­stable oscillator. Its output, pin 3, will nor­mally be close to 0V and this will turn on transistor Q2 via its 1kΩ base resistor. Q2 will then feed the +12V battery voltage through the neon tube’s inverter and it will light up. This is the situation with no audio input. If a negative trigger pulse is applied to pin 2 of IC1, its output at pin 3 will go high, turning off Q2 and therefore the neon tube. The length of time the output is high is controlled by the 150kΩ resistor and 0.1µF capacitor connected to pins 6 and 7. With these values, the time the output is high is 15 millisec­onds, after which the output pin will go low again. This turns Q2 on again and the neon lights up once more. Semiconductors 1 555 timer (IC1) 1 BC549 or equivalent NPN transistor (Q1) 1 BD140 PNP power transistor (Q2) 1 1N4001 or 1N4004 power diode (D1) Capacitors 1 100µF 25VW PC electrolytic 1 10µF 25VW PC electrolytic 4 0.1µF MKT polyester 1 .01µF MKT polyester Resistors (0.25W, 1%) 1 330kΩ 2 6.8kΩ 1 150kΩ 1 1kΩ 1 10kΩ The PC board should only take a few minutes to assemble. It can be housed in a small plastic case. Fig.3: use this diagram when wiring up the PC board. Fig.4: actual size artwork for the PC board. May 1997  25 You simply hook the little PC board in series with a 12V neon tube to make the light pulse on and off in time to the bass beat of the music. PC board. Use one of these pig­tails for the link. If you wish to use an IC socket, fit it next along with trimpot VR1, then fit the MKT capacitors and transistor Q1. Finally, fit the electrolytic capacitors and transistor Q2. If you use PC pins solder them in now and if you used an IC socket, plug the IC into it, making sure it faces the right direction. Testing Fig.2: this is the waveform at the collector of Q1 when the circuit is fed with a low-frequency audio signal. This negative trigger pulse comes from Q1’s collector via a 0.1µF coupling capacitor. To recapitulate, audio signals from the subwoofer are shaped and clipped by Q1 then applied to the trigger input of IC1. The negative-going edge will trigger IC1, turning Q2 and therefore the neon tube off. After a short time IC1 will reset and the neon will ignite again. As the audio input is taken from the subwoofer feed, only the low frequencies are present and these tend to be a repetitive beat effect. If normal full range audio were to be applied, the 555 would be triggered continuously. Thus, Q2 would be permanently held 26  Silicon Chip off and the tube would never be lit. The circuit as shown is capable of driving two neon tubes in parallel at its output. Diode D1 provides protection against accidental reversing of the 12V supply. Board assembly It is always wise to check the etching of the PC board before you begin any assembly. Look for open circuits (breaks) in the tracks or areas where the copper pattern may not be fully etched away. Any repairs needed should be done first. Begin by fitting all the resistors and diode D1. Solder each lead in turn, then cut off all the pigtails below the You can use a 12V battery or a DC power supply for the test. Connect the supply negative wire to the PC board earth. Connect the tube’s red wire to the neon tube + terminal on the PC board and the tube’s black wire to the PC board earth. When you connect the 12V positive lead to the +12V battery input on the PC board the tube should light. If it doesn’t, check the orientation of IC1 and Q2, then double check the wiring as detailed above. Once you get the tube to light, connect your subwoofer signal to the audio terminals, making sure that the signal wire is connected to the top of VR1. This done, set your car stereo to the normal listening level and adjust VR1 until the neon pulsing effect suits you. Don’t turn the control up too far or the tube will spend most of its life turned off. Also heed the warning on the tube and don’t run it continuously for more than two hours and probably for even shorter periods in very hot SC weather.