BJT LED Flasher Circuit Question - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: February 12, 2018, 01:14:30 pm
content: Hi,I made the attached circuit, but am getting unexpected results. I can't visually detect the LED flashing unless I swap the .33uF for something larger. In my circuit, C1 looks best at a minimum of 3uF. The only difference between the schematic and what I've got is Q1. The schematic is using a 2N2907 whereas I'm using a 2N3906.I get a frequency of 5Hz from my adaptations. It's 45Hz when I use the original values.I was wondering if the difference could be explained by the different transistor model.Thanks in advance,Carl

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 12, 2018, 02:54:43 pm
content: Based on the schematic alone, I would not expect this circuit to oscillate.

author: 0xfede
author_rank: Regular Contributor
timestamp: February 12, 2018, 03:25:08 pm
content: Quote from: Wimberleytech on February 12, 2018, 02:54:43 pmWell, it will oscillate but the poor LED life will be miserable...Attached to the post there are both the simulation picture and the LTSPICE project file. You should try changing transistor and other parameters yourself and then rerun the simulation.Best,0xfede

author: eev_carl
author_rank: Regular Contributor
timestamp: February 12, 2018, 03:35:10 pm
content: I got this working by using a different LED using the .33uF values in the schematic. I had a super bright LED in at the start. Other LEDs gave me the period (2 flashes / sec) described in the book.This is a picture of the scope for reference.

author: eev_carl
author_rank: Regular Contributor
timestamp: February 12, 2018, 03:38:16 pm
content: Thanks for posting the spice file. I've only worked with ngspice. Do I need the standard.dio and standard.bjt files locally or are they part of LTSpice if I download it?

author: 0xfede
author_rank: Regular Contributor
timestamp: February 12, 2018, 03:43:19 pm
content: Quote from: eev_carl on February 12, 2018, 03:38:16 pmI don't remember where all the models come from but I can share my LTSPICE library if you need it. It's about 100 MB in size so I cannot post it here.Best,0xfede

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2018, 04:16:04 pm
content: Quote from: 0xfede on February 12, 2018, 03:25:08 pmYes, add a resistor in series with the LED, to limit the current.

author: Damianos
author_rank: Frequent Contributor
timestamp: February 12, 2018, 08:47:39 pm
content: This book is for the bin for sure!!!Except (somewhat) of the Q1 base current, all others are uncontrolled...

author: floobydust
author_rank: Super Contributor
timestamp: February 12, 2018, 09:39:09 pm
content: Forrest Mims? The schematic looks like one of his, but bastardized.OP where is this crappy circuit coming from?

author: eev_carl
author_rank: Regular Contributor
timestamp: February 12, 2018, 09:41:09 pm
content: The Forrest Mims Circuit Scrapbook Volume II p. 8

author: eev_carl
author_rank: Regular Contributor
timestamp: February 12, 2018, 09:46:01 pm
content: The circuit doesn't draw much more than 3mA with VCC at 9V. I have a multimeter in series with the LED, coming from VCC.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 12, 2018, 10:02:37 pm
content: Quote from: floobydust on February 12, 2018, 09:39:09 pmAt my first look, I didn't think it would oscillate...but it does. I am not impressed by the design, however. I ran the sim provided by Oxfede and it looks like the NPN will see a lot of stress. There are better oscillators to choose from than this IMHO.

author: floobydust
author_rank: Super Contributor
timestamp: February 12, 2018, 10:03:34 pm
content: OK, looking at other Forrest Mim's oscillator circuits, he is missing a resistor to limit base-drive current, since.... 1976This is the two-transistor configuration I've always had success with, note it gets some success from inductive loads (transformer, loudspeaker).Another possibility is a multivibrator.edit: had wrong osc sch

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 12, 2018, 10:13:24 pm
content: Here is the complementary version using a bulb instead of an LED...also Mims.https://www.circuitlab.com/circuit/fc45k3/light-flasher/

author: eev_carl
author_rank: Regular Contributor
timestamp: February 12, 2018, 10:54:55 pm
content: Quote from: floobydust on February 12, 2018, 10:03:34 pmI'll check out the dual one. I have that in one of the other Mims books.