Going back to school - LED Flasher board - Page 1
Forum section: electronics / beginners

author: AstromechJapan
author_rank: Newbie
timestamp: October 20, 2024, 11:30:08 am
content: I am recreating an LED flasher board I did about 30 years ago.Still very much a beginner in calculating valuesI am using 2 different LEDs.LED Group 1:(Blue) Vf 3V If 20mALED Group 2:(Red) Vf 2.1V If 20mAIf my math serves me right, for LED Group 1, I will need a resistor value of 345 Ohm (actual resistor will be 348 Ohm as I can't find 345 Ohm)And for LED Group 2, I will need a resistor value of 300 Ohm.The capictors are 10uF 60V (aluminum electrolytic).The transistor is an NPN type (BJT 40V max, 600mA collector, Emitor Base voltage 6V, collector base voltage 75V...)What I am not sure how to calculate is the resistor rating for R3 and R4. At the moment, I have 10kOhm resistor in the diagram, but not sure if this is right?Is 10kOhm a proper value for R3 & R4?Any other potential issues with this diagram?(source voltage is a standard 9V battery)

author: madires
author_rank: Super Contributor
timestamp: October 20, 2024, 12:48:15 pm
content: Please check outhttps://www.petervis.com/GCSE_Design_and_Technology_Electronic_Products/2-led-flasher/2-led-flasher.htmlfor the classic circuit and a simple calculator.

author: Xena E
author_rank: Frequent Contributor
timestamp: October 20, 2024, 01:09:03 pm
content: Hello.OK, your calculation for the current only really applies where there's one LED,.As you have the circuit the 20mA or so will be shared between the group of LEDs.If the LEDs are mismatched then the brightness will vary.Also the astable mvb circuit transistors are shorting the 20mA to the LEDs which I'm pointing out only because it's an unusual way to do this, although it will "work'.The base resistors will give ≈ 1mA of drive so that won't be a problem, but what you select there in combination with the capacitors will determine the period of flashing.Using a 9 volt supply will mean that at each switching event the base emitter junction of the transistor will be reverse biased, (limit 7volts), it will most likely survive, however not good practice.Moving on to madires link, if you want to drive two groups of four LEDs, I would recommend using an emitter follower stage and use individual limiter resistors for each LED.X

author: AstromechJapan
author_rank: Newbie
timestamp: October 23, 2024, 06:05:52 am
content: Thank you both for the valuable info! This really helps - especially the equation for the value of the resistors!With regards to the transistor, @Xena E you mention that it should work, but will potentially burn out. To rectify this, do you recommend the Vebo be equal to input voltage (so 9V)?

author: AstromechJapan
author_rank: Newbie
timestamp: October 23, 2024, 10:08:21 am
content: Thanks for the great advice. I have updated the schematic and hopefully, I have not made any major issues here. I still need advice though on the transistor as all examples of a circuit like this I see online I see 9V battery but a 547/548 or 2222 NPN BJT transistor... again, re-learningI've added a resistor for each LED as recommended and for the 2 resistors, I have made them a 50KΩ resistor... which should give me just over 1/2 second flashing time.C=10uFR=50KΩ(Thank you Madires for that great calculation tutorial!)Here is the new schematic.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 23, 2024, 10:31:13 am
content: 1.If you assume 20mA per LED than you assume 80mA per transistor.With 9V supply and 50k (while only one is 50k) you will have base current about 8/50=0.16mA.To drive 80mA having 0.16mA in base transistor have to have beta=80/0.16=500.In general it is not good idea to assume having transistors with such high beta.2.You need not to calculate serial resistors for LED so accurate. 20% current deviation will not be noticeable.3.Having 9V you can use LEDs two in series reducing the collector current twice.4.30 years ago typical LED to be visible really needed 20mA. Nowadays I drive LEDs with 1..2mA. When I used 5mA small 3mm red led really gave in the eyes.