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Shopping For A WWVB Synced VFD Tube “Nixie” Clock

After much hesitation (partially from them be listed as coming from “Moscow, Russia, Canada”), green 7-segment VFD tubes in small preamp tube format were purchased from eBay. The datasheet arrived via email, being about a page and a half of mostly white space, a part drawing without labeled segments, a short list of characteristic voltages and currents, and an even shorter list of absolute max ratings. From the specs, it does not seem like they need AC driving of the cathode filament. They should be here within two weeks to confirm this.

For the WWVB receiver circuits, suitable parts were located at Digi-Key, in decent stock quantities and prices. C-Max makes a range of low frequency tuned ferrite coil antennas with very small footprints, for under $3, as well as a demodulation IC suitable for use with the antenna, the CME6005. A module based on the IC is about $9, the IC itself is under $3.

Current plan for controlling the seven segment anodes and the grid on each tube, as well as interpreting the demodulated WWVB signal, is a large pin count AVR driving darlington transistor arrays. Multiplexing will be avoided in order to use the tubes at maximum illumination. The cost of a large pin-count AVR is offset by not having to purchase latches needed to drive the tubes at full duty cycle with less GIO. The TQFP-100 parts look suitable.

Information on NIST’s WWVB atomic clock synchronization transmitter in Boulder, CO is here.

Following are linked Digi-Key part numbers and datasheets for the C-Max WWVB parts.

561-1002-ND C-Max CMA-60-19 60KHz 19mm Ferrite Coil Antenna (datasheet)
561-1014-ND C-Max CMRR-6D-60 CME6005 Based 60KHz Receiver Module (datasheet)
561-1013-1-ND C-Max CME6005 60KHz AM Demodulator IC SSOP-16 (datasheet)

Posted by rmrubin Posted in: Electronics Comments Off September 2007

Mathless LED Driver Tested

Broken SD Cardreader replaced, picture taking capabilities restored. Most will probably agree by the quality of the pics that soon is the time for a new camera.

The circuit was changed, replacing the R2 of 1K for a 10K. This drops the current required for the reference transistor, Q1, to under 200uA, an insignificant level compared to the LED current draw.

Testing was done with a LM317/LM337 based cigar box PSU, and a Fluke 110 DMM. The assembled circuit acted as expected, and as simmed by LTSpice. The Vbe of the reference transistor, Q1, was tested as being .65V, giving a forward current of slightly under 10mA with a 68R resistor for R1. Current, and thus brightness, was consistent from 3V to 18V, althought higher voltages are possible while still keeping the LED driver transistor, Q2, within its absolute maximum power dissipation spec.

Possible Mathless LED applications include dynamic input LED lighting for people with no clue about electronics or even basic math, or in an academic lab as a current source experiement.

Posted by rmrubin Posted in: Electronics Comments Off September 2007

Very Low Cost Mathless LED Driver

Intended for people with interests in LED lighting, but are allergic to the math needed for calculating current limiting resistors, and have very little money. A good example would be an art student or custom car person, although it its ease of use could be appreciated by anyone. If you’re not so allergic to math, the equation is:

R_led=(V_input-Vf_led)/If_led

The device should regulate current through an LED, with a wide range of input voltage. The design uses a pair of PNP transistors as a current source with a target output of 10-15mA with 3-20V the target. Simulation in LTSpice predicts the circuit is sane. Dissipation in the LED drive transistor is under 200mW, and should be within specification of the MMBT3906 used to 155F ambient temperature.

BOM: