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Along with a DMM, I consider a logic probe to be a basic tool for anyone developing digital circuits. A logic probe is a pencil-like device that can be used to check different parts of a board and return a visual/audio signal that indicates whether or not a signal is logic high, low, or tristated (or not connected to a driver or a pulled up or pulled down input). There are usually two controls on a logic probe. The Pulse/Continuous switch will cause a light-emitting diode (LED) to ash when a changing signal is encountered or
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APPENDIX D
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Probe Tip
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Figure D.15 An LED would seem to be able to provide the basic functions of a logic probe.
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a single LED to light for a continuous level. Along with an LED, many logic probes have a speaker in them that will provide beeps and tones so that you can tell what is going on without looking at the LEDs. The speaker audio output is a feature I highly recommend getting. The other switch is TTL/CMOS and selects which voltage threshold should be used. Normally TTL is 1.4 V, and CMOS is 2.5 V. Good-quality logic probes can be bought for as little as $20, and I recommend that you buy one rather than try to build one yourself. You may think that you could build one yourself with a resistor and an LED like the circuit shown in Fig. D.15. The circuit in Fig. D.15 has three problems:
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1 It doesn t detect high or low outputs, just high. 2 If a circuit doesn t have enough current capacity, the LED won t light. 3 There is no indication on level transition.
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To detect high, low, and tristate/no connections, another LED will have to be added to the circuit. If the single LED is not lit, then the pin could be attached to a low, tristate, or low-current high output. To x this, voltage comparators capable of driving a LED should be used in the circuit, as shown in Fig. D.16. In this circuit, if the probe is driven high or low by a pin, one of the LEDs will light. A switchselectable threshold reference voltage divider could be put in for a TTL/CMOS selection.
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Probe Tip Vcc R R
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Figure D.16 Adding a comparator creates a logic probe circuit with the capabilities of detecting high and low logic levels.
MISCELLANEOUS ELECTRONIC REFERENCE INFORMATION
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Probe Tip Vcc R R
P-Channel MOSFET - On When Probe is Connected to LOW Voltage
Figure D.17 Enhanced logic probe circuit that uses a P-channel MOSFET to detect lowvoltage conditions.
The circuit as it stands will not detect open conditions; for this feature, some kind of MOSFET sensor is required. A P-channel MOSFET could be put in to sense when the logic level is low, as shown in Fig. D.17. When the probe pin is low, the P-channel MOSFET will turn on, passing current to the LED. To add a transition output, a single shot should be triggered by each of the LED drivers. The modi cation to the logic probe circuit shown in Fig. D.18 will cause a third LED to ash when the LED drivers become active. Looking at this, you re probably con dent that you could build this circuit easily. You probably could, although there may be some problems with getting the circuit to t in a small enough package to be useful. In addition, if you did attempt to build one yourself; it probably would cost as much in parts as buying a completed unit. For these reasons, I recommend that you avoid the headaches of building one and just buy one ready made. As unlikely as this sounds, the Radio Shack logic probe contains all the features I ve listed here and is quite inexpensive (it is $20), making it the device you should look at rst. I ve owned three of them over the years they are as good as or better than other logic probes costing twice as much or more from more prestigious manufacturers.
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