Hi Michael, Please allow me to briefly describe how that part of the circuit works. I will be referring to the schematic diagram which you can find here<http://www.saltechips.com/products/thermneon/hard-software.html> .
As you know, the length of the illuminated bargraph is a function of the current flowing through it. In the schematic area J-15, N1 is the IN-13 bargraph tube. The length is proportional to the current through terminals A(anode) and K(cathode). Although the physics of the glow discharge inside the tube (primer here<http://www.saltechips.com/products/thermneon/theory.html>) will cause the tube to appear to function with a PWM signal, the error in displayed value will be unacceptable. Therefore the tube must be operated with a constant, or slowly varying current. Not PWM. The thermNeon achieves this constant drive current by utilising an 'operational amplifier constant current sink'. The circuit is formed by IC3, Q3, R17 and P1. IC3 is trying to make the voltage on the top leg of R17 (emitter of Q3) equal to the voltage on its pin 3. By doing so, a current is allowed to flow through R17, P1, Q3 and hence the cathode terminal. The value of the current is approximated by Ohm's law: I(tube) ≈ V(pin 3) / (R17+P1). So far the circuit described takes a voltage on pin 3 of IC3 and turns it into a current through N1. The voltage on pin 3, which ultimately controls the length of the bargraph) is generated by the main microcontroller IC2, R14 and C10. IC2 generates a PWM signal on pin 13. The duty cycle of the is varied by the firmware according to the desired length of illumination. R14 and C10 form a low-pass filter, which effectively takes the average DC value 'contained' in the PWM signal output from IC2. So there you have it, the bargraph length starts from a firmware instruction, turns into a PWM duty cycle, is averaged by a low-pass filter, drives a current sink which finally drives the IN-13 producing a beautiful neon glow :) Hope this answers your question. Alex On Thursday, March 28, 2013 6:58:59 AM UTC, mikegregg wrote: > > Neat. Thanks for the opportunity. > > I saw the one that Dave at EEVblog received. > > I like your design. > > One question. How are you adjusting current in the bar graph? Are you > doing that with PWM? > > Michael- > > On Wed, Mar 27, 2013 at 6:33 PM, Stuckey > <[email protected]<javascript:>> > wrote: > > Hello - > > SALTechips has donated a thermNeon kit to a giveaway via > TubeClockDB.com. To > > enter, all you need to do is leave a post. > > > > > http://www.tubeclockdb.com/forum/Notices/4889-Win-a-free-thermNeon-Kit-from-SALTechips%21.html > > > > > Disclosure - I own TubeClockDB.com > > > > Brian > > > > -- > > You received this message because you are subscribed to the Google > Groups > > "neonixie-l" group. > > To unsubscribe from this group and stop receiving emails from it, send > an > > email to [email protected] <javascript:>. > > To post to this group, send an email to > > [email protected]<javascript:>. > > > To view this discussion on the web, visit > > https://groups.google.com/d/msg/neonixie-l/-/ugOHEpwZT1sJ. > > For more options, visit https://groups.google.com/groups/opt_out. > > > > > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msg/neonixie-l/-/xzc1tir3U-0J. For more options, visit https://groups.google.com/groups/opt_out.
