Introduction
Most students and experimenters can't afford an HP545 Logic Probe which is something over $200. This simple kit performs the same functions and at the same time may also be used as a clock. Unlike the $200 HP545 this is not encased in a nice package with a menacing steel tip probe with power clips that seem to always be snapping off just as you are trying to observe a logic level. But, at 1/100th of the price, this kit does a nice job.
Kit Description.
This is a very simple kit consisting of only three components that an experimeter may quickly wire on a solderless breadboard to test and debug a design. It allows the experimenter to quickly determine whether a point is at a logic one, zero, open or a pulse train. There is no life threatening steel tip. Rather, the user moves a wire from point to point.
The "probe" circuit consists of a programmed PIC12C508A in an 8-pin DIP, a single 100K resistor and an LED with a built in 330 Ohm resistor. Just three components.
When the probe input is at logic zero, the LED is off. When at a logic one, the LED is full on and when open, the LED is dimly lit.
Positive transitions above 20 pulses per second cause the LED to flash at 10 pulses per second. We have verified operation to 10.0 MHz.
In addition two output leads are provided which may be used as clock sources. These supply nominally 1.0 and 10.0 pulses per second. A 330 Ohm series limiting resistor is recommended to protect the PIC if a clock output is accidentally connected to +5 or to ground is and two 330 Ohm resistors are supplied with the kit. This feature may eliminate the clutter of a 555 which many beginners use as a clock.
Note that the unit may be used as both a probe and a clock at the same time.
Text Schematic.
1 +5VDC 2 10 pulse per second clock output 3 no connection 4 1 pulse per second output 5 100K to terminal 6 - This is the probe input 6 other side of the 100K resistor 7 anode of LED 8 GRD and cathode of LED with built-in 330 Ohm resistor
Pricing.
As with all publications and kits, my intent is to provide educational materials which enable others to learn and do things they might not otherwise be able to do. My intent is not to make money, but to break even. However, this concept of having my undergraduate students develop materials to help others has worked very well in motivating and raising the self esteem of my students. And, hopefully, many people outside of Morgan have benefitted. In my mind, it is money well spent.
I think this probe and clock is a real plus to anyone who does not currently have a logic probe. This includes hobbyists and students and I would like to get as many out there as possible with mortgaging the house or asking my wife to take a second job. Thus, it is priced to break even or perhaps lose a bit. Please let others know of this probe kit.
Many companies have incorporated this into their designs. The source code may be purchased fro $100.00.
Credits.
The idea for this circuit and a good amount of the design effort was performed by H. Paul Roach, a '97 graduate of Morgan. For two years Paul volunteered his time for free. I am fortunate to have Paul on a fully funded basis until June, when he returns to the island paradise of Trinidad.
But, a lot of other people made this possible.
In the Spring, '97, LaQuieshia Smith pioneered experimenting with the PIC and without the benefit of any emulator, she really did some amazing things. She graduated with the singular honor of being valedictorian, winner of the Department's Leadership Award and the IEEE Outstanding Student Award. LaQuieshia is now a graduate student an Penn State.
Over the summer, Paul was joined by Towanda Malone and Nicole Ambrose who worked very hard to interface the PIC with all manner of peripherals with the idea of presenting this on the Web for the benefit of everyone. They did exceptionally well.
However, their success was in large measure due to the generosity of the XEROX Corporation. They support the idea of students developing educational materials to help others and their generosity enabled me to quickly obtain emulators and many other tools that we would not have had for this summer effort. In fact, I don't know where I would be without their help.