These are the answers to the EQ questions that appeared in Circuit Cellar 258(January 2012).

Answer 1—Q1 and Q2 are wired as a differential amplifier, but in this digital application, it’s probably better to think of them as a current switch. Whenever VIN < VTH, the current through R1 is shunted to ground, and whenever VIN > VTH, the current passes instead through Q2 to the base of Q3, turning it on. As long as Q1 and Q2 are reasonably well matched, VTH is the input voltage at which the switchover occurs, and this is relatively stable with respect to temperature. For example, if VIN is being driven by standard TTL logic, you might set VTH to 1.5 V.

Answer 2—R1, combined with VTH, sets the amount of current flowing through Q2 to drive the base of Q3. This current should be sufficient to drive Q3 into saturation, given the expected load on it (R3 plus the external circuit). R2 serves to make sure that Q3 isn’t turned on by any leakage current through Q2 when it’s supposed to be off. For example, a value of 100 kΩ would bypass currents of up to 6 µA or so.

Answer 3—When VIN is high, there is virtually no current flowing through the input terminal—just the leakage current through Q1’s base-collector junction. When VIN is low, the driving circuit must sink the current set by R1 divided by the beta (current gain) of Q1. For example, if R1 is 4,300 Ω, giving a current through Q1 of about 1 mA, and the beta of Q1 is 50, then the driving circuit must sink about 20 µA.

Answer 4—Better: The component count is reduced by one transistor. Better: VTH is stabilized by the forward drop of a diode, making it less dependent on the exact value of the positive supply. Worse: The switching voltage is now determined by the combination of D1 and the base-emitter drop of Q2, both of which vary with temperature. Worse: Now the driving circuit must supply all of the base current for Q3 when it is in the high state.

Contributed by David Tweed (eq at circuitcellar.com)

# Electronics Engineering Crossword (Issue 259)

The answers for the crossword puzzle published in Circuit Cellar 259 (February 2012).

# Solid-State Lighting Solutions Project

Electronics system control, “green design,” and energy efficiency are important topics in industry and academia. Here we look at a project from San Jose-based Echelon Corp.’s 2007 “Control Without Limits” design competition. Designers were challenged to implement Pyxos technology in innovative systems that reduced energy consumption. Daryl Soderman and Dale Stepps (of INTELTECH Corp.) took First Prize for their Solid State Lighting Solutions project.

The Pyxos chip is on the board (Source: Echelon & Inteltech)

So, how does it work? Using the Pyxos FT network protocol, this alternative lighting project is a cost-effective, energy-efficient solution that’s well-suited for use in residential, commercial, or public buildings. You can easily embed the LED lighting and control system—which features SSL lighting, a user interface, motion detectors, and light sensors—in an existing network. In addition, you can control up to five zones in a building by using the system’s fully programmable ESB-proof touchpad.

Another view of the Pyxos chip is on the board (Source: Echelon & Inteltech)

This winning project, as well as others, was promoted by Circuit Cellar based on a 2007 agreement with Echelon.

|

# audioXpress: HP456A Current Probe Restoration

Retro electronics (or “retronics”) projects are growing in popularity. Across the globe, professional engineers and DIYers alike are tweaking, updating, and hacking retro systems to create all sorts of innovative designs. Restoring and upgrading an old electronics tool, MCU-based design, or audio system can be a rewarding experience.

In the February 2012 issue of audioXpress magazine, Bill Reeve details how he restored a Hewlett-Packard 456A current probe (“Restoring the HP 456A Current Probe”). Here’s an abridged excerpt:

The restoration is finished and ready for cover installation (Soure: Bill Reeve AX 2/12)

The Hewlett-Packard 456A AC current probe is a treasure. It can be bought cheaply because many of the units sold were battery powered and all were designed with a now-out-of-date oscilloscope interface connector. However, when restored, the 456A is a fabulous addition to any test bench, matching the performance of more expensive modern instruments.

Released as a new product by the Hewlett-Packard Company in 1960, the 456A was HP’s first solid-state, stand-alone, clip-on current probe. Its elegantly designed amplifier uses two— then “state-of-the art”—PNP germanium transistors.

The Original Probe
In 1960, The Hewlett-Packard Journal (July-August, Vol. 11) proudly announced:

“This new probe measures current over the full range of the frequencies most commonly used in typical work—25~ to 20 megacycles—and over an amplitude range from below 0.5 mA to 1 A rms…The probe operates with an accompanying small amplifier…to convert the AC current being measured to a proportional voltage. This voltage can then be measured with a suitable oscilloscope or voltmeter. The current-to-voltage conversion factor is 1 mV/mA.”

The 456A operating and service manual is available at www.hparchive.com, but this scanned copy contains incorrectly annotated schematic values for R7 (should be 3300 Ω), R8 (should be 2700 Ω) and C5 (should be 0.01 μF).

Old battery-powered 456As are usually in excellent physical shape because when their batteries ran down these instruments were often shelved and forgotten. Another 456A advantage is that its probe head is wired directly to the amplifier, so they cannot be separated by surplus electronics dealers.

Restoration
Restoration of the 456A consists of three steps: replacing the old battery pack with DC power, restoring the amplifier electronics, and converting the obsolete oscilloscope banana plug interface to a BNC connector.

Step 1: Replace the old battery pack. Remove the two Phillips-head screws on the housing back to slide off the 456A’s cover. Re-thread the screws into the frame to keep them from getting lost. ….

Step 2: Restore the amplifier electronics. At this point, if you are happy with your current probe’s performance, you can skip the following upgrades, but these are five modifications you might need to perform to get your 456A working or improve its performance:

• Replace the electrolytic capacitors
• Replace the two germanium transistors
• Replace the 8-V breakdown diode (CR1)
• AC-couple the output
• Flow solder onto the printed circuit traces

Photo 6 is an annotated close-up of the amplifier’s single-sided printed circuit board. Following vacuum tube circuit convention, the +5 V is labeled “B+” and the –8 V is labeled “B–”. There are three electrolytic capacitors in the amplifier (see the horizontal silver cylinders in Photo 6), and their replacement is straightforward. ….

Photo 6: The amplifier's original printed circuit board (Source: Bill Reeve AX 2/12)

Step 3: Convert the oscilloscope interface to a BNC connector. This final modification can be performed one of three ways. Pomona electronics (visit the website pomonaelectronics.com) sells a female banana to male BNC adapter (Model 1296). You can cut the banana plug connector off the existing cable and attach a male BNC connector. This requires special tools.

You can replace the output cable with coax having one BNC end. This is a straightforward replacement. Photo 9 shows the new BNC output cable. …

Photo 9: BNC ouput cable installed (Source: Bill Reeve AX 2/12)

This restoration should make your 456A ready for another 50 years of service.

Note: The complete article appears in the February 2012 issue of audioXpress magazine. audioXpress magazine, like Circuit Cellar, is an Elektor group publication.

|

# RFI Bypasssing

With GPS technology and audio radio interfaces on his personal fleet of bikes, Circuit Cellar columnist Ed Nisley’s family can communicate to each other while sending GPS location data via an automatic packet reporting system (APRS) network. In his February 2012 article, Ed describes a project for which he used a KG-UV3D radio interface rigged with SMD capacitors to suppress RF energy. He covers topics such as test-fixture measurements on isolated capacitors and bypassing beyond VHF.

Photo 2 from the Febuary article, "RFI Bypassing (Part 1)." A pair of axial-lead resistors isolate the tracking generator and spectrum analyzer from the components under test. The 47-Ω SMD resistor, standing upright just to the right of the resistor lead junction, forms an almost perfect terminator. (Source: Ed Nisley CC259)

Ed writes:

Repeatable and dependable measurements require a solid test fixture. Although the collection of parts in Photo 2 may look like a kludge, it’s an exemplar of the “ugly construction” technique that’s actually a good way to build RF circuits. “Some Thoughts on Breadboarding,” by Wes Hayword, W7ZOI, gives details and suggestions for constructing RF projects above a solid printed circuit board (PCB) ground plane.

You can read this article now in Circuit Cellar 259. If you aren’t a subscriber, you can purchase a copy of the issue here.