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A New Neon T for your nex O ur last neon tube modulator for cars (SILICON CHIP May 1997) proved quite popular, but what do you do if you don’t have a sub-woofer output from your stereo system to drive it? This latest version solves that problem. As well, the original unit turned off the neon tube for each bass beat but some readers wanted the neon to fire on the beat. This unit can do both. Oh! You do have a sub output on your system but you’d like to use this updated circuit? No worries, just omit three capacitors and off you go. How it works As you can see from the circuit (Fig.1) and photograph of the PC board there isn’t a lot to it. One IC, three transistors, a couple of diodes and a small handful of resistors and capacitors and its done. The Left and Right channel inputs are fed via a 0.47µF electrolytic capacitor to the inverting input of IC1a and they appear at the output, pin 1, as a summed inverted signal. The 0.47µF capacitor along with the 10kΩ input resistor value form 32  Silicon Chip a single-pole high-pass filter; the response will roll off at 6dB/octave below 33Hz. Trimpot VR1 controls the gain of IC1a over a 16:1 range from 0.33 to 5.3. The non-inverting input, pin 3, is connected to the 6V rail and this voltage is propagated through IC1b to the base of Q1. IC1b is configured as a 2-pole lowpass filter set to roll off at 150Hz. Its response, together with the 33kΩ resistor at its output and the .047µF capacitor, give a flat response from by Rick Walters very low frequencies up to 150Hz where the output is 3dB down. The response falls at 18dB per octave above this frequency. Thus the frequencies at the base of Q1 are predominantly those above 20Hz and below 200Hz (see filter response Fig.5). If you wish to use the circuit in conjunction with a subwoofer, you do not need the low-pass filter based on IC1b. The easiest way to eliminate this filter is to simply leave out the three capacitors (0.1µF, .047µF and .022µF) associated with it, although it will make very little difference to the display either way. With the DC level at the base of Q1 being in the vicinity of +6V and its emitter also at +6V, it will normally be turned off, but any signal with an amplitude above 6.6V will turn it on. Our bass beat note does just that for us. IC1c and IC1d are wired as a monostable (one stable state). With no input signal, pin 13, the inverting input of IC1d, sits at about 5V due to the voltage divider action of the 120kΩ resistor to 6V and the 470kΩ resistor to ground. As the inverting input is at a lower voltage than the non-inverting input (pin 12), the output, pin 14, will be near 12V. The 47kΩ and 82kΩ resistors hold the inverting input of IC1c at +7V which ensures that its output, pin 8, is near ground. Each of these outputs is connected to the base of an N-channel FET (Field Effect Transistor) via a www.siliconchip.com.au Tube display xt soundoff! soundoff! 4.7kΩ resistor to hold it either normally on (Q2) or normally off (Q3). Thus the neon tubes connected to pin 2 on the output connector will normally be illuminated and those connected to pin 3 will normally be extinguished. When the base of Q1 is taken more positive than the emitter it will turn on and the collector voltage will fall from 12V to 6V. This 6V negative step will be transferred to pin 9 of IC1c, via the .01µF capacitor, and pull it down from 7V to 1V, thus causing pin 8 to rise to near 12V. This will turn Q3 on and also raise pin 13 of IC1d from 5V to around +17V, via the .082µF capacitor. Thus its output will fall to ground and Q2 will turn off. The capacitor at the collector of Q1 will rapidly discharge but the potential at pin 9 will remain near 0V as pin 14 is now also close to 0V. The outputs will stay in their triggered (unstable) states while the .082µF capacitor slowly discharges. When the voltage at pin 13 falls below 6V pin 14 will revert to its high state (12V), causwww.siliconchip.com.au ing pin 9 to revert to 7V and thus pin 8 will fall to 0V (the original stable state). This time delay is set by the .082µF capacitor and the parallel value of the 120kΩ and 470kΩ resistors. Diode D2 and the 68kΩ resistor hold the base of Q1 high, keeping it turned on and preventing any audio signals from re-triggering the monostable. To enable you to test the PC board without any neon tube being connected we have provided a LED to mimic the tube’s response. It is wired in parallel with the normally on neon The unit doesn’t have a case: we left that part up to you because every installation will be different. Some may simply heatshrink the PC board and conceal it under the dash with some cable ties to hold it in place. November 2001  33 Fig.1: the complete circuit has just one IC, three transistors, a couple of diodes and a small handful of resistors and capacitors. LED1 is not shown on this circuit: see the wiring diagram. tube and it should light when power is applied to the PC board. A small terminal strip has also been fitted to the board edge to allow easy connection of the battery, the LED and the neon tube leads. Putting it together The first step is to check the copper pattern on the PC board against the magazine artwork, looking for bridges between tracks or cuts in the tracks. While such defects are unusual in commercially made boards it is easier to check for such before the PC board is assembled. The next step is to fit and solder the resistors and diodes. Use a multimeter to check the resistor values as sometimes, depending on the body colour, the band colours can be very difficult to identify. Follow with the IC socket (if used) or the IC. Ensure pin 1 faces towards the Zener diode. Now add the trimpot, the capacitors and lastly, the terminal strips and FETs. Solder three wires, two for the audio inputs and one for the earth, to the PC stakes. That completes the PC board assembly. We have deliberately not mounted the PC board in a case because every installation will be different. In most cases, we imagine the board will be “hard wired” into the car’s electrical system, in which case the board could simply be heat-shrunk and secured up under the dash with a cable tie. On the other hand, some constructors may wish to mount the board in a case (perhaps for portability), complete with RCA or similar sockets for the input. We’ll leave this side of it completely up to you. Testing it. Connect the LED anode (longer lead) and cathode to the second and fifth terminals on the terminal strip. Connect the positive 12V lead either from a power supply or car battery to the seventh terminal and the negative lead to the first. Connect an audio source to the input – just one channel is fine for testing. The best source of audio is a tape recorder or CD player because when you (shortly!) connect the neons up, you’ll probably find the RF interference they generate will obliterate any nearby radio signal! 34  Silicon Chip www.siliconchip.com.au Fig.2: the component layout on the PC board. The position of the LED shown here is in the “normally off” position – moving the cathode to terminal 2 should make it normally on. While listening to the music adjust the trimpot until the LED flickers off in time with the beat. You’ll need a reasonable amount of level to make the circuit work – if the LED stays on (ie, doesn’t flicker) wind the wick up on your stereo and/or set the pot at its maximum. You may have to reset this control to accommodate the different input level when you fit it in your vehicle. For comparison, here’s a same-size pic of the completed PC board, with the pattern shown below. If you swap the LED over to the fourth and fifth terminals, the LED should be flickering on (as distinct from flickering off) in time with the beat. Finally, the LED can be removed or it can be left in circuit – it doesn’t matter either way. If you wish to dress it up with a bezel, it Parts List – Neon Tube Modulator 1 1 2 1 2 1 PC board coded 05111011, 75mm x 46mm plastic box 59 x 109 x 34 (Jaycar HB-6025 or equivalent) RCA chassis mounting sockets 3-way terminal strips (Jaycar HM-3173 or equivalent) 2-way terminal strips (Jaycar HM-3172 or equivalent) neon tube (pair), Jaycar ST-3130 (red) or ST-3134 (blue) and/or ST-3138 bicolour neon tube 2 5mm x 3mm threaded spacers 2 3mm x 8mm countersunk bolts 2 3mm nuts Semiconductors 1 LM324 quad op amp (IC1) 1 BC338 NPN small signal transistor (Q1) 2 MPT3055E N-channel MOSFETs (Q2, Q3) 1 1N5404 3A diode (D1) 1 1N914 small signal diode (D2) 1 3mm or 5mm red LED (LED1) Capacitors 2 100µF 25VW PC mounting electrolytic 1 0.47µF 25VW PC mounting electrolytic 1 0.1µF MKT polyester 1 .082µF MKT polyester 1 .047µF MKT polyester 1 .022µF MKT polyester 1 .01µF MKT polyester Resistors (0.25W, 1%) 1 470kΩ 1 120kΩ 1 82kΩ 2 33kΩ 2 10kΩ 1 6.8kΩ 1 1kΩ 1 50kΩ trimpot (VR1) www.siliconchip.com.au 1 68kΩ 2 47kΩ 1 36kΩ 2 4.7kΩ1 3.3kΩ1 2.2kΩ could make a neat dashboard indicator, showing the pulses the neons are working to! Won’t work! If it doesn’t appear to work, the first check is to make sure that you actually fitted the LED with the correct polarity. The LED should be lit when across terminals two and five. Shorting the drain of Q2 to its source should cause the LED to light. If it does not, either the LED is faulty or in backwards. Once you get the LED to light with the short it should stay lit with the short removed. Check that the voltages on pins 9 and 13 are as shown on the circuit. If this is not the case, check each resistor value around IC1c and IC1d and check your soldering. Once the monostable is working you can check the audio with a multimeter set to read AC volts. Table 1: CAPACITOR CODES Value IEC Code EIA Code 0.1µF   100n   104 .082µF  82n  823 .047µF  47n  473 .022µF  22n  223 .01µF  10n  103 November 2001  35 The circuit was originally designed to run with this Neon Tube set from Jaycar. Its wiring is shown below (fig.3). Again, moving the black lead from the No 3 terminal to the No 2 terminal turns the neons from normally on to normally off. If you wish to use the Jaycar ST-3138 Neon (photographed below), you’ll need to open up the case and make a modification – adding an extra wire, as shown in the photo at the bottom of the page. Fig.4 shows the wiring to the PC board. Starting at the audio input and tracing through the circuit, pin 1 of IC1a should typically have a signal of around 1VAC with a normal input and VR1 fully clockwise and depending on the program material, a slightly lower voltage at pin 5. The same voltage should be measured at pin 7 of IC2b and a little less at the junction of the 33kΩ resistor and the .047µF capacitor. As long as you get a reading at each point you need not worry too much about the exact value. Careful checking of your work should show up the problem. The board is capable of driving 8-10 ST-3130/4 neon tubes so you can wire some normally on and others normally off to get the exact effect you want. Go out and knock them out at your next sound-off. For the serious car buff! While browsing the Jaycar catalog looking for the part number for the above tube we came across another one that intrigued us, the ST-3138. 36  Silicon Chip The extra wire (blue) is soldered to the switch in the position seen here. See the text for a more detailed explanation. It features a 3-position switch which can select colour 1 (C1), colour 2 (C2) or fade from colour 1 to colour 2 and back, on a continuous basis (CC). While the fade function didn’t work very well on our sample, our idea was to have one colour selected and let the beat change it to the other colour, then after the delay the tube would revert to the first colour. You will have to be a bit adventurous if you want this type of display as the tube’s electronics are in a sealed plastic case. A hobby knife www.siliconchip.com.au around the edge soon had the lid off. Our unit had two wires, one red, one white joining pads on the PC board to the switch. The red wire went to the position on the switch marked C2. By selecting C1 this red wire was grounded. Selecting C2 switches to the pink neon colour which changes to blue when the red wire is grounded. Just what we need! The cigarette lighter plug was chopped off and the striped black lead run to the battery supply (terminal 2). The plain black lead was connected to the battery negative (terminal 7) along with the battery lead. A wire was soldered to the red wire termination and taped to the black fig.8 lead. It was terminated on the normally off terminal (6). This is all shown clearly in the photograph. There is no reason why you can’t Table 2: RESISTOR COLOUR CODES    No.  1  1  1  1  2  1  2  2  1  2  1  1  1 Value 470kΩ 120kΩ 82kΩ 68kΩ 47kΩ 36kΩ 33kΩ 10kΩ 6.8kΩ 4.7kΩ 3.3kΩ 2.2kΩ 1kΩ 4-Band Code (1%) yellow purple yellow brown brown red yellow brown grey red orange brown blue grey orange brown yellow purple orange brown orange blue orange brown orange orange orange brown brown black orange brown blue grey red brown yellow purple red brown orange orange red brown red red red brown brown black red brown mix and match both types of neons. The main limitation is the current capacity of D1 (3A maximum). By AUDIO PRECISION 2HD-FREQ AMPL(dBr) & LEVEL(dBr) vs FREQ(Hz) 10.000 05 JUN 100 10:00:03 5.0000 0.0 -5.000 -10.00 -15.00 -20.00 -25.00 -30.00 20 100 1k 5-Band Code (1%) yellow purple black orange brown brown red black orange brown grey red black red brown blue grey black red brown yellow purple black red brown orange blue black red brown orange orange black red brown brown black black red brown blue grey black brown brown yellow purple black brown brown orange orange black brown brown red red black brown brown brown black black brown brown connecting the positive leads of the neons to the battery positive terminal instead of through the diode, the limitation becomes the voltage drop across each FET. With an on resistance (RDSon) of 0.15Ω you can safely draw 2-3A through each FET without a heatsink. With a decent heatsink you could probably double this. We measured the current consumption of the samples and found that the pair of ST-3134 tubes drew 250mA and the bicolour tube drew 130mA on pink and 90mA on blue. Armed with this information you can figure out your display requirements and how you will have to wire SC the tubes. Fig.5: The filter gives a flat response from very low frequencies up to 150Hz where the output is 4dB down. The response falls at 18dB per octave above this frequency. Thus the frequencies at the base of Q1 are predominantly those above 20Hz and below 200Hz. UM66 SERIES TO-92 SOUND GENERATOR . THESE LOW COST I.C.’S ARE USED IN MANY TOYS, DOORBELLS AND NOVELTY APPLICATIONS   1-9 $1.10 10-24 $0.99 25+ $0.88 EACH INC GST www.siliconchip.com.au November 2001  37