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What do you do for a LED flasher now that the LM3909 is no more? LM3909 Replacement Modules Module: by Thomas Scarborough it’s even more versatile! The LM3909 was a legendary IC, which the designers (National Semiconductor) modestly described as a “LED Flasher/Oscillator”. Its popularity was surely due both to its great simplicity and versatility. It could flash a LED off a wide range of voltages, at a wide range of frequencies. It could also flash LEDs in parallel, could produce a tone in a loudspeaker, trigger a Triac or pulse an incandescent bulb – among other things. Sadly, though, the LM3909 has been discontinued and is now very difficult to find. T he module shown here is designed to do just about everything that the original LM3909 did – and more! There are a couple of differences – the most obvious one is that the module is quite a bit larger than the DIP-sized LM3909. Supply voltage is much more usable 3V-18V, compared to the LM3909’s 1.15V to 6V. Current consumption may be as low as 100µA, rather than the LM3909’s typical 0.55mA. Pulse width may be controlled more easily than it could with the LM3909. 34  Silicon Chip And not least, this circuit can pulse two LEDs alternately. It will also serve, among other things, as a quartz clock driver and continuity tester. Rather than present a host of similar circuits, however, a single module is shown here, along with a table showing how the module can be used in a variety of ways. Circuit description IC1a is a Schmitt RC oscillator or “clock generator”. Only the capacitor, C1, can be regarded as a fixed part of the oscillator; R1, R2, RX, RY and D2 are all components which can be changed to allow the module to perform in different ways. The output of the oscillator charges and discharges capacitor C2 through IC1b, connected as an inverting buffer. The charge on C2 then controls IC1c. When the output of IC1c is combined with the the output of IC1d (which is the inverted output of IC1a), brief pulses are sent in opposite directions between IC1c and IC1d. Depending on their direction, these pulses cause either LED1 or LED2 to flash. siliconchip.com.au Two of the many possible versions of the LM3909 Replacement Module – on the left, the alternate LED flasher with LINK2, RX at 4.7MW, RB at 1kW, no R2 and RB (hidden behind capacitor at top) at 47kW. At right is the LED/bulb flasher, with LINK3, RX at 2.2MW, R2 at 470kW (in series with its diode), RB at 1kW and no RY. Note the MOSFET lamp driver is also in place on this PC board. For most of the time, LINK1 stays in place (the circuits won’t work without it). RX sets the frequency of the flash, while RY sets the pulse width (or “on” time) of one or both of the LEDs. Resistor RB limits the current through the LEDs to safe values. LINK1 is a switch which either enables (when connected to +V) or disables the oscillator (tying pin 1 to 0V via R1). In all except one case, LINK 1 stays in place unless you want to stop the circuit oscillating. In fact, LINK1 could be replaced by a switch if you want to make it even more convenient. LINKS 2 and 3 can be changed to make their respective gates operate in different ways, in turn affecting the operation of the module. It is this which gives the module significantly D1 1N4001 K 100 µF 25V + Q1 IRF540 IC1: 4093 (ENABLE/ DISABLE) 1 IC1a 2 A C1 470nF +3-18V* * VALUES SHOWN ARE FOR 12V SUPPLY LINK1 100kΩ (R1) A 3 13 Rx K D2 1N4148 12 5 R2 Ry 14 11 LAMP D G S IC1d Rb IC1b 4 6 LINK2 LINK3 K A C2 470nF LED1 8 9 λ LED2 λ K IC1c A 10 OUTPUT TO PIEZO, CLOCK, ETC 7 0V 1N4148 A SC 2008 1N4004 K A LEDS K LED/LAMP FLASHER K A IRF540 D G D S Where the LM3909 had just about everything inside the DIP package, the replacement module requires a few more components – but it does more than the LM3909 ever did! siliconchip.com.au more flexibility than the LM3909 it is replacing. LINK2 is used where a short pulse width is required and LINK3 is used where a square wave is required. Various possibilities are shown in the table overleaf. Component values in this table are selected for 12V operation and will likely need to be modified for other supply voltages. One of the other features of this module which you didn’t get (as much) with the LM3909 is that it allows significant experimentation and modification of values. With the exception of RB, changing any of the resistors (even going down to 0W) will not cause any damage to the module (RB limits LED current through the LEDs and should never be less than about 470W). Finally, if LEDs are wired in parallel, these should best have individual current-limiting resistors, the combined resistance of which should not be less than about 330W. As mentioned earlier, one of the features of this module is its wide supply voltage range (3-18V). This is connected via a terminal block on the left side of the module which is in turn protected against incorrect polarity by diode D1 and is decoupled (smoothed) by the 100mF capacitor. This capacitor is specified as 25V to allow up to an 18V supply; if you are never going to use a supply greater than 12V, a physically smaller 16V capacitor can be used. September 2008  35 Q1 Ry + 100 µF K A D2 IC1 4093B 1 4148 EV- A LINK2 LINK1 + 1 1 8 0 8 0 0 11N4001 +V (3-18V DC) 0V 470nF R1 C1 Rx LINK3 K 100k 470nF LED2 D1 LAMP C2 LED1 pm a l Rb IRF540 G D S P MAL RE HSALF OUTPUT (PIEZO, MOTOR, ETC) R2 The same-size photo above matches the component overlay at right. The photo is of the Alternate LED Flasher. While this PC board might look like a double-sided type, it’s not: it was produced in a panel which included a double-sided board so pads also appear on the top side, along with pads and tracks on the bottom side. The lamp, its terminal block and the lamp driver (MOSFET Q1) are optional – if you don’t want to drive a lamp, simply leave them out. Construction This project could hardly be simpler – simply mount the components as shown on the overlay, also using the photograph as a guide. Start with the terminal blocks. For most uses a two-way block will suffice on each side of the PC board (the four-way on the left side is only required for the incandescent lamp driver). Follow these with the three header pin sets (for LINKS 1, 2 and 3), then the resistors and capacitors, next the LEDs, the MOSFET (if required) and finally the diode D1 and IC1. The diode D2 could be left out if you don’t want to build the lamp driver or modified alternate LED flasher but given its low cost, it might as well be included. Without R2 in place, it will have absolutely no effect. Note that all components except the resistors and the two “block” capacitors are polarised – the circuit won’t work if you put them in the wrong way around (and you could damage them). Also be very careful when soldering in components with close lead spacing (especially the IC). It’s very easy to bridge across adjacent pads and once again, this will stop the project working and could cause damage. And if you want to experiment with different values, here’s a tip: solder in some PC stakes for all resistor values which you might want to change (R2, RB, RX and RY). It’s a lot easier to tack resistors across the stakes rather than solder them into the PC board and take them out again (besides, it’s easy to damage the PC board tracks with too much soldering and desoldering). This shows how the two outputs from IC1d (yellow) and IC1c (green) add to give double the drive signal to the LEDs (white trace). 36  Silicon Chip Parts List – LED Flasher Module 1 PC board, 62 x 50mm, code 11009081 3 2-way PC-mount terminal blocks 3 2-way header pin sets 1 4093 quad Schmitt NAND gate 1 IRF540 MOSFET (Q1 – optional) 1 1N4004 power diode (D1) 1 1N4148 small signal diode (D2) 2 5mm LEDs, colours as required 1 small incandescent lamp, voltage to suit supply voltage (optional) Capacitors 1 100mF 25V electrolytic capacitor 2 470nF MKT metallised polyester capacitors Resistors (0.25W, 1%) 1 100kW (R1) Other resistors to suit application – see component selection guide The flasher board set up for a 50% duty cycle flasher. In fact, it is not quite 50%, due to the differing positive and negative switching thresholds in the gates. siliconchip.com.au LED Flasher Modules – Component Selection Guide RX RY RB R2 Links LEDs LED Flasher 2.2MW 47kW 1kW None LINK 1 IN LINK 2 IN LED1 Alternate 4.7MW 47kW 1kW None LED Flasher LINK 1 IN LINK 2 IN LED1 LED2 Micropower 4.7MW 10kW 2.2kW None LINK 1 IN LED1 Alternate LINK 2 IN LED2 LED Flasher Square Wave 4.7MW None 1kW None Alternate LED Flasher LINK 1 IN LINK 3 IN Notes: Ultrabright LEDs are required here. A 47kW resistor is wired in series with the power supply’s +V. The circuit draws about 100µA. Remove resistor R1 to minimise current drain. LED1 LED2 Modified 2.2MW None 1kW 470kW LINK 1 IN LED1 Square Wave and diode LINK 3 IN LED/Bulb Flasher The 470kW and diode are wired in parallel with RX. Depending on the orientation of the diode, the LED will be illuminated longer or shorter than half of a complete cycle. By making IC1d pin 11 power a MOSFET, this configuration may be used to flash an incandescent bulb. Modified 4.7MW 47kW 1kW 1MW LINK 1 IN LED1 Alternate and diode LINK 2 IN LED2 LED Flasher The 1MW and diode are wired in parallel with RX, yet the diode’s polarity is immaterial here. The effect is a pulsing of the two LEDs “in twos”. Quartz Clock Motor Driver RY and RB may need to be altered, depending on the characteristics of the quartz clock’s stepper motor. This is merely an experimental circuit, since an RC timer will not provide good time keeping. 10MW 150kW 470W None LINK 1 IN Stepper variable LINK 2 IN motor Externally Pulsed None 150kW 470W None LINK 1 IN Stepper Quartz Clock LINK 2 IN motor Motor Driver The external pulses need to match the supply voltage of the module. These may need to be further lengthened. This may be done by wiring a diode between the source of the pulses and IC1a pin 2, with the cathode to pin 2 and a resistor in parallel with C1 (try 2.2MW). Continuity Tester 2.2kW None None None LINK 1– Piezo The piezo sounder is wired to the wired to 0V sounder sounder outputs for LED1. The via a 1MW continuity tester’s leads are taken resistor from the LINK1 terminals. R1 can be (see note) increased to 1MW for the continuity tester – alternatively 1kW to avoid obtaining a signal for high impedance SC continuity. siliconchip.com.au September 2008  37