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3-State Logic Probe B ack in the November 1998 edition of S ILICON C HIP, we described a very handy 3-LED Logic Probe. The circuit is just as viable today as when it was published six years ago and literally thousands of kits have been sold. That’s no surprise: a logic probe is one of the “must have” test devices in any hobbyist’s, technician’s or even engineer’s test equipment armoury. What’s more, it’s cheap – so it’s an ideal beginner’s or school project (not to mention one that will come in very handy over the years)! So why re-invent perfectly good wheels and present it once again? Simple: one of the suppliers of the afore-mentioned kits, Altronics, reasoned that the it could be be made even better by re-designing the PC board to a handier shape, adding a few extra (low-cost!) components to provide better input protection, moving the supply on-board and finally, housing the probe so it was much more like . . . a probe! (The original project was housed in a small plastic case which was a little unwieldy to use. It also had 84  Silicon Chip clip leads to connect to power on the device under test. The newer design doesn’t have a case at all: it’s housed in heatshrink. But we’re getting a little ahead of ourselves.) And the best part of all – it’s even cheaper. With no case to worry about, the total cost of the new design has been kept at less than ten dollars. Yep, go without one packet of coffin nails and you can buy yourself a logic probe kit! All right, what’s a logic probe? As its name suggests, a logic probe is a device which indicates any logic state at its input probe. Now that makes sense, doesn’t it? Of course, there is just a little more to it than that. First of all, the logic level should only be low (at or very close to ground) or high (at or very close to the positive supply). But a faulty device can have an output level somewhere around between these limits, or even bouncing back and forth between them. Ideally, then, a logic probe should be able to indicate all three circuit states – high, low and something else – and that is what this simple design does. It has three LEDs which are readily visible, located near the top of the probe. The green one indicates a low level, the red one a high level and the yellow one is lit whenever the level changes from high to low. You may wonder why we bothered with the yellow indication. We have just stated that if the level is low, the green LED will light, if the level is high the red one will be lit, and if the level is changing from high to low then obviously both will light. The fault condition described above can sometimes cause both LEDs to come on and this would give us a false indication. The yellow LED needs a full high-low transition to light it, thus eliminating any false indication. How does it work? As you can see from the circuit there is not much to it. A 4001 quad 2-input NOR gate is used as the logic level sensing device and also the LED driver. This particular chip also lets us make a monostable by cross-coupling two gates. We’ll get to why we want that in a moment. So let’s start at the input. The probe siliconchip.com.au IDEAL SCHOOL PROJEC T! One IC, three LEDs and a sprinkling of other components are all it takes to make a versatile Logic Probe. At left we are using the probe to check out the very first project ever published in SILICON CHIP, a 1GHz Digital Frequency Meter from November 1987. Yes, it still works perfectly! Original Design by Rick Walters tip is connected to pins 5 & 6 of IC1b via an input protection circuit consisting of two 10kΩ resistors and a 16V zener diode. This will protect the rest of the circuit from very high level inputs voltages – up to around 300V – though what you would be doing using a logic probe with this level of input we’re not sure. Still, for the sake of two resistors and a zener it’s worthwhile protection. The 10MΩ resistor holds the gate inputs low and prevents their input capacitance being charged and staying high when the probe encounters a momentary high level. The output of IC1b is fed to pins 1 & 2 of IC1a which in turn, drives the LEDs. Since each gate effectively inverts its input and there are two gates, signal inversions via these gates, the output of IC1a is in phase with the input. Thus when the input is low, the output of IC1a is low and the green LED will be lit. When the input goes high, the green LED will go out and the red one will light. The output of IC1b is also coupled through a 100nF capacitor to one input of IC1c. This input is held low by the 10kΩ resistor to ground. IC1c’s output, pin 10, is coupled via a 180nF capacitor to the inputs of IC1d. These inputs are held high by the 100kΩ resistor which means the output at pin 11 will be low. A low to high transition at the output of IC1b will pull pin 8 of IC1c high and consequently pin 10 will go low. This will pull pins 12 & 13 low, taking pin 11 high and thus turning on LED3. As pin 11 is also connected to pin 9 of IC1c, it will hold the output of IC1c low even after the initial logic signal at pin 4 has charged the 1nF capacitor. The yellow LED will stay lit until the voltage on the 180nF capacitor, which is charging through the 100kΩ resistor, reaches the switching threshold of IC1d. When it is reached, the output of IC1d will go low, the yellow LED will extinguish and the output of IC1c will go high again. There are a few minor differences in this early prototype but the overall setup is the same . Obviously, this shot was taken before the heatshrink “case” was applied. siliconchip.com.au August 2004  85 Watch the polarity of the IC, diodes, LEDs and Zener. Thus each high to low input transition will flash the yellow LED for 18ms. At low frequencies this is readily apparent but as soon as the input frequency is high enough, the LED will appear to be lit continuously. So to sum up, if the green or red LED is on, the circuit being measured is indicating a valid logic condition (ie, low or high), although if you want a high and you get a low you obviously have a problem. A yellow LED on may mean a fault or it may mean a pulse train – either way, you know there is something to investigate. Power for the Logic Probe is “onboard”: a pair of button cells in series gives 6V. Diode D1 protects the logic probe if you accidentally put the cells in around the wrong way. The voltage drop across this diode means that the supply is closer to 5V than 6V. Note that the “ground” clip lead must be connected to the ground or 0V of the circuit in order to give the logic probe its ground reference. PC board assembly The new PC board is deliberately made as small as possible to make it a comfortable fit in the hand. Once assembled, the board is covered with a length of heatshrink tubing, leaving uncovered only the LEDs, battery and on-off switch at one end and the probe at the other. The assembly details for the Logic Probe are quite straightforward. Start with the resistors and capacitors, as none of these are polarised. The battery holder and on-off switch are next, soldered directly the appropriate pads on the PC board. Next come the three LEDs – make sure they are not only in the right place, but the right way around – and finally the 4001 IC. The IC, like everything else, is soldered directly to the PC board (ie, no socket) as this keeps the height at a minimum. We used a probe from an old multimeter lead as the input prod but failing this, a nail or a small gauge screw with a filed point could be pressed into service. We’re sure your ingenuity won’t fail you here. Testing Insert the batteries into their holder and turn the on-off switch to on. The green LED should immediately light. If it doesn’t, you have a problem somewhere in the circuit (dry joint, bridged track, etc) which needs to be found and fixed. Use your multimeter to measure the voltage at pin 3 of IC1a. It should be at ground potential, ie, 0V. Now short the probe to the probe’s positive supply using a short length of wire or clip lead. This should extin- Once the probe is built and tested, cut the heatshrink to an appropriate length (ie, LEDs to probe) . . . 86  Silicon Chip Parts List – 3-LED Logic Probe 1 PC board, 20 x 133mm, coded K-2586 (Altronics) 1 length 30mm heatshrink tube, ~100mm long 1 miniature slide switch, SPDT (SPST also acceptable) 1 battery holder, PCB mounting, to accept two CR2016 cells, 1 length black hookup wire, ~250mm long 1 length red hookup wire, ~50mm long 1 insulated alligator clip 1 probe similar to multimeter probe (see text) 2 small cable ties Semiconductors 1 4001 quad NOR gate 1 1N4148 or similar Silicon diode 1 16V, 1W Zener diode 1 red 3mm or 5mm LED 1 green 3mm or 5mm LED 1 yellow 3mm or 5mm LED Capacitors 1 180nF polyester 1 100nF polyester 1 1nF polyester Resistors 1 10MΩ 3 10kΩ 1 100kΩ 3 1kΩ . . . and shrink it with a heat gun on low setting (a hair drier also works, just not so quickly). siliconchip.com.au . Where from, how much . . . While the original design remains the copyright of SILICON CHIP, this PC board pattern was developed by Cameron Costigan at Altronics and this particular kit (K-2586) is available exclusively from Altronics stores, mail order (1300 797 007) or web (www.altronics.com.au) for just $9.95 plus p&p. guish the green LED and light the red one. As you remove the probe from the supply, you should see the yellow LED flash briefly. Tap the wire to the probe a few times until you see it. In use This view is of the back of the PC board showing the battery mounting. Naturally, the battery must not be covered by heatshrink! It really is as simple as connecting the ground clip to the 0V (or ground) of the circuit under test, applying the probe and noting the LED colour. For testing most 5V logic circuits, the 6V supply of the probe will be very close to perfect, especially as 0.6V will be lost across the protection diode. Therefore logic high and low will be correct. If you want to use it on a logic circuit with, say, a 12V or 15V supply rail, the logic levels for high and low will obviously be different. In some cases, a “low” may be above the probe’s threshold and falsely give a “high” reading. In this case, we suggest you revert to the arrangement used in the origi- nal circuit and take the supply from the circuit under test. That way, the logic thresholds will move to track the supply. You can use any supply rail up to 15V. Provision is made on the PC board for attaching external supply lines. If you use an external supply you should first remove the on-board batteries. The probe will work with most logic devices, particularly the now-prettystandard CMOS chips (“C” and “HC” devices), as well as older TTL chips. The upper frequency depends on the supply voltage: with the on-board batteries it should be good for up to about 3MHz or so; with a 15V supply perhaps 8-9MHz. SC And finally, the finished probe, complete with ground connector and heatshrink “case”. siliconchip.com.au August 2004  87