I’m designing a simple SW receiver for the 20m band and need a rudimentary frequency indicator activated by the multiturn rotary knob. The knob is part of a recuperated Pritt gluestick mechanism pushed into service as a PTO-drive for the VFO. The idea is to use reed switches to power the indicator LEDs on the frontpanel. On the glueholder a stick is glued which holds a magnet at the end. As the knob is turned, the magnet advances over 6 reed contacts, each in turn activating a corresponding LED on the frontpanel. Of course, as soon as the magnet is advanced, the active LED is switched off and there is no indication for the duration of the next cycle until the next LED is turned on. Is there a way to make it so that the last LED remains lightened until the next is switched on? Of course this also need to work in reverse. Thanks beforehand for your help!


You are trying to describe a very Rube Goldberg type of mechanism for something that should be done in a much more uncoupled manner. You should come up to speed what microcontrollers are all about. Have the microcontroller (MCU) able to detect position of the knob (which can now be a simple potentiometer feeding an A/D converter OR a rotary encoder feeding quadrature waveforms to two inputs). The MCU would also control the display and run the VFO. You tie all this together with software that can easily solve the problem of when the display should change in relation to how the rest of the things change.

  • \$\begingroup\$ Thank you for your prompt and knowledgeable reply! I'm sure there are more elegant ways to solve this kind of problem but since this receiver is aimed at the beginner with basic knowledge of analogue electronics, solutions involving microcontrollers or Arduino are out of the question. It's ironic that you call my proposed mechanism a Rube Goldberg type of solution in this respect... \$\endgroup\$ – Erik Devriese May 22 '18 at 13:19
  • \$\begingroup\$ @ErikDevriese - You may want to revise your perception regarding microcontrollers. Things like the Arduino and the Raspberry Pi have completely changed the landscape of what beginners that barely understand electronics are doing these days. \$\endgroup\$ – Michael Karas May 22 '18 at 14:29

For each reed switch you need a D type latch and specifically you need a D type latch that has a clear pin. For all D type latches you wire the "D" inputs to logic 1. For the D latch that is directly associated with a particular reed switch, the reed switch feeds its clock input.

This means that one side of the reed switch is tied to logic 1 and the other side that connects to the clock has a (typically) a 1 kohm to 10 kohm pull-down resistor.

That clock input also connects to the other D latch clear inputs via individual diodes to each clear pin. Ensure that there is a pull-down resistor on each clear input.

Step and repeat for all reed switches and D latches.

Or just use a microcontoller and software to control the LEDs

  • \$\begingroup\$ Is PR set to 5V for every D-flop? What is the value for the pull-down resistors in series with the diodes? Can I simply connect the the LEDS via a resistor to the Q-outputs from +5V supply? Thanks! \$\endgroup\$ – Erik Devriese May 22 '18 at 14:49
  • \$\begingroup\$ Which chip are you thinking of that has a PR pin? Pull down resistors are not series but from clear input to 0 volts and 1 k to 10k. What LEDs are you thinking of using? \$\endgroup\$ – Andy aka May 22 '18 at 17:46
  • \$\begingroup\$ This site explains the workings of a D-flop. It does not mention a particular chip: instructables.com/id/D-Flip-Flop-With-Preset-and-Clear I plan to use standard red LEDs. Sorry, my digital experience is very shallow! \$\endgroup\$ – Erik Devriese May 22 '18 at 18:06
  • \$\begingroup\$ I just want to be sure what you mean by a "PR" pin. \$\endgroup\$ – Andy aka May 22 '18 at 18:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.