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3-LED LOGIC PROBE Ever been chasing a problem on a digital logic board and wasted a lot of time because you were too lazy to get the scope out and plug it in? What, you don’t even own one? This logic probe will prove invaluable in digital fault finding and only costs a few dollars. By RICK WALTERS All right. So what is a logic probe? A logic probe is a small hand-held device which indicates the logic state at its input probe. The logic level should only be ground (low) or at the positive supply (high) but a faulty device can have an output level somewhere around half the supply. Ideally, a logic probe should indicate all three circuit states and that is what this simple design does. The probe has three LEDs which are readily visible whether you are right or left-handed. The red one indicates a low level, the green 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 red LED will light, if the level is high the green 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 of Fig.1 there is not much to it. A 4001 quad 2-input NOR gate is used as it lets us make a monostable by cross-coupling two gates. We’ll get to that in a moment, so let’s start at the input. The probe tip is connected directly to pins 5 & 6 of IC1b. The 10MΩ resistor holds those pins low and prevents the input capacitance being charged and staying high when the probe en­ counters a momentary high level. The output of IC1b is fed to pins 1 & 2 of IC1a which in turn, drives the LEDs. Note that since each gate effectively inverts its input and there are two signal inversions via these gates, the output of IC1a is in phase with the input. Thus when the input is low, the Silicon Chip’s Electronics TestBench  35 Fig.1: the circuit uses a 4001 quad 2-input NOR gate to indicate high, low or fault logic conditions. output of IC1a is low and the red LED will be lit. When the input goes high, the red LED will go out and the green one will light. The output of IC1b is also coupled through a .001µF capaci­ tor to one input of IC1c. This input is held low by the 10kΩ resistor to ground. IC1c’s output, pin 10, is coupled via the 0.18µF 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 .001µF capacitor. The yellow LED will stay lit until the voltage on the 0.18µF 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. Thus each high to low input transition will flash the yellow LED for 36 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 just to sum up, if the red or green LED is on, the logic circuit being measured is indicating a valid condition (ie, low or high), although if you want a high and you get a low you ob­viously have a problem. Power for the Logic Probe comes from the circuit being measured and can be anywhere between 5V and 15V DC. Diode D1 protects the logic probe if you accidentally make the wrong supply connections (ie, wrong polarity) to the circuit. PC board assembly We made the PC board as small as possible, so you could fit it into a smaller case than the one we used, if you have one. We would have preferred a slightly narrower rectangular case but the one we used is readily available and inexpensive. On the positive side, if you have large hands, the size and shape of the speci­fied case is quite convenient to handle. The assembly details for the Logic Probe are shown in Fig.2 and are quite straightforward. Don’t use an IC socket for the 4001 as there is Silicon Chip’s Electronics TestBench Fig.2: not shown on this wiring diagram are the positive and negative supply leads which clip onto the circuit being measured. Fig.3: actual size artwork for the PC board. not much depth in the case we have specified. Use the PC stakes as they are a convenient connection for the LED leads. Keep the wires close to the PC board when you solder them and cut the top off the stakes or else they will prevent you from assem­bling the case properly. Drill the three holes in the case for This is the view inside the Logic Probe case. Note that the leads to the three LEDs must be sleeved to avoid the possibility of shorts. the LEDs and file a notch in the end panel to bring the power wires out. Make it small enough so that the wires are lightly clamped when the case is screwed together. We secured the board inside the case by using a small self-tapping screw into one of the integral pillars. But the pillar is very short and you must be careful not to tighten the screw too much otherwise it will penetrate right through the case. If you look closely at the inside photo of the Logic Probe you will note that we have placed a black fibre washer underneath the screw head to avoid this problem. Another point to note about the inside photo is that the LEDs should have sleeving on their leads to avoid A slot is cut in one of the end pieces of the case for the power supply leads. the possibility of shorts. 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. I’m sure your ingenuity won’t fail you here. Testing Connect the power leads to 5-12V and the red LED should immediately light. If it doesn’t, you probably have its leads reversed. Don’t worry though, just make the connections correctly and it should work properly. Use your multimeter to measure the voltage at pin 3 of IC1a. It should be at ground potential; ie 0V. Now put the probe on the positive supply. This should extinguish the red LED and light the green one. As you remove the probe from the supply, you should see the yellow LED flash briefly. Tap the probe on and off a few times until you see it. The beauty of this device is that if you connect it to a logic PC board with a 5V supply, all the functions work as de­scribed. But it can be connected to any supply up to 15V with safety and the logic thresholds will move to track the supply. It will work with all “C” & “HC” devices as well as the older TTL range. The upper frequency depends on the Parts List 1 PC board, code 04104981, 50 x 26mm 1 small plastic case, Jaycar HB6030 or equivalent 1 red crocodile clip 1 black crocodile clip 3 5mm LED bezel clips 8 PC stakes 1 6mm long self-tapping screw 1 fibre washer (see text) 0.5m red hookup wire 0.5m black hookup wire Semiconductors 1 4001 quad 2-input NOR gate (IC1) 1 1N914 small signal diode (D1) 1 5mm red LED (LED1) 1 5mm green LED (LED2) 1 5mm yellow LED (LED3) Capacitors 1 0.18µF MKT polyester 1 0.1µF MKT polyester or monolithic ceramic 1 .001µF MKT polyester Resistors (0.25W, 1%) 1 10MΩ 1 10kΩ 1 100kΩ 3 1kΩ supply vol­tage. With a 5V supply the 4001 should indicate up to 2-3MHz and around three times this frequency with a 15V supply. SC Silicon Chip’s Electronics TestBench  37