Problem with Astable Multivibrator Driving Circuit - Page 1
Forum section: electronics / projects

author: FBMinis
author_rank: Regular Contributor
timestamp: November 24, 2021, 06:41:34 pm
content: I breadboarded an Astable Multivibrator Driving Circuit which makes two LED blink at the rate I want but when I use an LED stick it looks dim, not fully bright.I started with this theory:https://www.electronics-tutorials.ws/waveforms/astable.htmlThen put together this circuit (Falstad):https://tinyurl.com/yh48yt8n(LED blink at around 1Hz, transistor are 2N3904 and I have tried some BC547 and 2N2222 with the sam result)I would like to replace the red LEDs with an LED stick. These work at 5V and seem to pull close to 350mA.When I replace the red LED+resistor with the LED stick, it blinks at the correct rate but it doesnt appear to be fully ON.Here is a video:https://youtu.be/dnoilRe5ucs(in this video they are blinking faster because I was testing different resistors)(I have some AO3400 N-Mosfets and AO3401 P-Mosfets here I that's the way)Thank you in advance.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 24, 2021, 07:50:10 pm
content: As shown in your linked schematic, you connect the LED in series with the 1k resistor. You're only going to get a couple mA through the LED when ON. Enough to get a regular red LED light up.How did you connect your LED stick?

author: FBMinis
author_rank: Regular Contributor
timestamp: November 24, 2021, 07:56:28 pm
content: I removed the resistor+red led and replaced this load with the led stick.I also replaced the transistor with 2N2222 as it has a higher maximum current but the stick isnt as bright as when I connect it directly to the powerbank.As far as I understand, it's getting enough base current to let the led stick current fully flow through but I dont think that's happening.Here is the circuit cimulation:https://tinyurl.com/yhoon5hc

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 24, 2021, 08:08:54 pm
content: As you must know, the collector current of NPN transistors is roughly proportional to the base current. You need to increase the base current. Different NPN transistors have different gains (basically hFE), but be aware the gain varies widely depending on each individual transistor and operating conditions...Here one way to increase the base current would be to lower the value of the 22k resistors (but you'll need to adjust the capacitors accordingly).You could also use darlington NPN transistors instead. But this basic astable circuit with NPN transistors as is is not ideal for "high" currents.One approach you could use here without redoing the whole circuit with MOSFETs would just be to add ONE MOSFET and drive its gate with one of the outputs of the multivibrator (collector of either NPN transistors, put a series resistor back from collector to 5V.

author: FBMinis
author_rank: Regular Contributor
timestamp: November 24, 2021, 08:22:22 pm
content: I thought the 1K resistors were acting as the base resistor for the led stick 2N2222I am going to try to lower the 22Kohm resistor and increase the capacitor size.Like this:https://tinyurl.com/yfyzvoan

author: FBMinis
author_rank: Regular Contributor
timestamp: November 24, 2021, 08:39:54 pm
content: QuoteThis circuit works but the led stick still isnt as bright as connecting directly to the powerbank. Im going to try the AO3400 mosfet.

author: CaptDon
author_rank: Super Contributor
timestamp: November 26, 2021, 01:02:23 am
content: You are probably very close to the knee voltage of that LED stick where an increase or decrease of even .1 volts will have a huge effect on light output. So you need a transistor or mosfet that can be fully saturated with almost zero loss from E to C or D to S. A mosfet will probably work better, but again you are now on the cusp of the gate voltage required for saturation. Some refer to a certain type of mosfet as a Logic Level device which will approach full saturation with 5 volts on the gate. This will probably give you the best result without raising the 5 volt supply to the LED stick up to perhaps 6vdc.

author: FBMinis
author_rank: Regular Contributor
timestamp: December 02, 2021, 10:18:14 pm
content: This is working fine for me with an AO3401https://tinyurl.com/y5t2v779