How can I convert this optocoupler led flasher circuit to switch a mosfet or bjt - Page 1
Forum section: electronics / projects

author: FBMinis
author_rank: Regular Contributor
timestamp: November 08, 2021, 10:48:43 pm
content: This simple circuit makes the LED flash at a "medium rate". I would like to flash the small LED strip you see in the background instead (it pulls 400mA at 5V)How can I modify this circuit to have a mosfet of transistor flash the LED strip?Falstad circuit simultation:https://tinyurl.com/yz6hodry

author: dmendesf
author_rank: Frequent Contributor
timestamp: November 09, 2021, 12:00:54 am
content: Make a darlington using the transistor inside the optocoupler and an external medium power BJT.

author: dmendesf
author_rank: Frequent Contributor
timestamp: November 09, 2021, 12:02:09 am
content: The 1K resistor must be made smaller.

author: pqass
author_rank: Super Contributor
timestamp: November 09, 2021, 01:07:59 am
content: What an odd flasher. See waveform below.Because your Vcc=5V you'll need a logic-level MOSFET capable of Id > 400mA; the one suggested was just my first Google hit. But, because the on-state of the flasher only reaches 3.1V, I don't think that would be enough to turn on the logic-level MOSFET directly. Therefore, I drive it with a common 2N7000. But because the duty-cycle is only 10%, you have to invert that twice if you want to preserve the same duty cycle.https://www.pcboard.ca/irlz44n-n-channel-mosfethttps://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l5YCcyWrQDhAVndAbIulgCwDMkixYA7BSMYttpNgKYC0YYAUMZNSE4AmTOhbsxISTwBOgsCPDFig0kKUqWYeNznDR4hZhYtitSNzB517NVFXrJ6gCasAZgEMArgBsALuw+rM7g9iawPADu8orikibc0RImDmHcAEqC8VnixMb2fEwpMFiJMZhcNkYaUGW26pWpYMq1AObyeFoKHVqxtUlWXVWKjRYDipKciI4JA9NShvNI6hZCdPXg8-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

author: FBMinis
author_rank: Regular Contributor
timestamp: November 09, 2021, 02:23:37 pm
content: Thank you very much for your timeAfter reading your explanation, and given that I only have some AO3400 N-Mosfets in sot-23 package here, I decided to try first with a BC517 Darlington transistor. It works well. Increasing the value of R3 to 1K and above keeps the LED strip dim, without pulsating. Values between 100 and 470 ohm give a nice, well defined ON-OFF pulse.https://tinyurl.com/yfwgto9dThe AO3400 has a gate threshold voltage of typical 1V and an Rds of a few miliohms, as far as I understand it could be turned fully On by the flashers output..http://www.aosmd.com/pdfs/datasheet/ao3400.pdf

author: rvalente
author_rank: Frequent Contributor
timestamp: November 09, 2021, 02:51:08 pm
content: Why not using a 555 as astable? Plenty of calculators online. The coupling tolerance factor for optocouplers are so wide that this circuit would change behavior from lot/vendor

author: FBMinis
author_rank: Regular Contributor
timestamp: November 09, 2021, 02:56:01 pm
content: Because I dont have any 555 at hand

author: rvalente
author_rank: Frequent Contributor
timestamp: November 09, 2021, 03:51:48 pm
content: So get 3 transistors and make a astable, any other known circuit is better than this

author: FBMinis
author_rank: Regular Contributor
timestamp: November 09, 2021, 04:44:37 pm
content: Quote from: rvalente on November 09, 2021, 03:51:48 pmNot the point of this thread but thank you anyway.----------------------Using the AO3400 N-Mosfet produced a similar waveform, the LED strip flashes as intended. In theory a 100K resistor between the gate and source should be present but in practice I saw little change. Anyway, it works and it's adequate to my needs, thank you.