Suggested device for switching LEDs ? - Page 1
Forum section: electronics / beginners

author: scottb721
author_rank: Contributor
timestamp: August 25, 2020, 07:39:30 am
content: An eye rolling subject so I apologise in advance.In an effort to make some largish active lighting, and to also just make something for a bit of fun, I've decided to have a crack at a large 'VU meter'.I've chickened out on building from scratch and ordered a couple of VU kits and plan to have them drive RGB strip LEDs.The strip light has 5 LEDs per 'group' with a common +24v rail and a switched ground for each colour of the group.The strip has it's own power supply so I only need to apply the LED switching to it.The plan is to have the VU kit switch a given colour in each section via the ground side, rather than its own kit LEDs, so I'm wondering if there's a suggested Mosfet or tranny for this?The VU kit drives 10 LEDs but I think I'll only use 5 LED groups but I don't know yet if the VU kit switches the high or low side for its o/p.Thanks

author: ledtester
author_rank: Super Contributor
timestamp: August 26, 2020, 04:13:41 am
content: How much current will a single transistor have to switch?How often will the transistors be turned off and on?What transistors do you already have in your parts collection?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 26, 2020, 09:38:38 am
content: To me this looks like a lack of general knowledge about how transistors work.Build some simple circuits on a breadboard. Switch some LED's, build an emitter follower, a schmitt trigger and other simple transistor circuits to build up your knowledge of transistors.

author: ledtester
author_rank: Super Contributor
timestamp: August 26, 2020, 12:49:14 pm
content: Quote from: Doctorandus_P on August 26, 2020, 09:38:38 amWell, this is the beginner's forum.

author: scottb721
author_rank: Contributor
timestamp: September 08, 2020, 11:47:01 am
content: Quote from: ledtester on August 26, 2020, 04:13:41 am8mA is the max per transistor and they'll switch fairly regularly, as often as the 3915 outputs will respond to audio input.The 3915 provides a GND O/P so I bought a couple of PNPs and mocked up a basic cct driving some LEDs. Still waiting for the VU kits to arrive.One thing I'm wondering though is whether the transistor can even be operated because the Base, driven from the VU kit, isn't referenced to the Collector or Emitter because the LEDs I'm switching, which the transistor would be in series with, are on their own separate cct.

author: ledtester
author_rank: Super Contributor
timestamp: September 08, 2020, 12:25:38 pm
content: QuoteA schematic would be helpful.

author: scottb721
author_rank: Contributor
timestamp: September 08, 2020, 01:16:55 pm
content: https://imgur.com/a/MaBFWwSThe LED cct has its own supply. Will just tie the 3915 cct to it.

author: Ian.M
author_rank: Super Contributor
timestamp: September 08, 2020, 01:34:54 pm
content: ALM3915bargraph IC is designed to provide user preset constant current drive (typ. 12 - 13 mA) for common anode LEDs with the anodes connected to a positive voltage less than its V+ supply. Its complicated by the LED on pin 11 also having a 100uA current source feeding it, used to help cascaded blanking in dot mode.Its also designed to provide soft transitions between LEDs so that in dot mode, at no time are two LEDs both hard on, and there are no gaps at transitions between LEDs where no LEDs will be on.This vastly complicates interfacing it to a 24V common anode LED strip. It may not be able to provide enough current to drive the strip directly, and odds are the VU meter kit uses a much lower V+ than 24V so direct connection would be prohibited anyway.Options for interfacing it to the strip depend on the strip's LED current, per color per unit length, and whether or not the VU meter and LED strip PSUs share a common ground.We need more data to give decent advice.

author: scottb721
author_rank: Contributor
timestamp: September 22, 2020, 10:08:54 am
content: Pretty happy with how it turned out. Was fun tinkering.Will make another for the Right channel and throw them into some diffused strip light housings.Running it off Zone 2 of my Amp which has a separate level control.https://www.youtube.com/watch?v=ydelrOHr7l0&feature=youtu.be

author: Ian.M
author_rank: Super Contributor
timestamp: September 22, 2020, 10:12:35 am
content: You don't get away without showing us the schematic of the interface circuit you developed . . . .

author: scottb721
author_rank: Contributor
timestamp: September 22, 2020, 10:41:56 am
content: Too funny. There was none really.I rejigged the VU pcb to allow me to fit PN200s where the LEDs would normally go and used some ribbon cable to wire each element of the strip.I found a 5v strip so changed to that so it could run off the same PSU, albeit a 12V one.I did add an additional dropping resistor for a single segment I was testing (each 'segment' is 3 LEDs in parallel with each LED having it's own resistor) to account for the 12V supply but this dimmed it too much. Ditching the extra resistor seems to not have fried anything and is well below the brightness seen when running the LEDs directly off 5V. Having said that I did have to dim the green sections with extra resistors to match the brightness .I also switched to LM3914 so there's more swing to the lights.None of it close to proper technique.

author: Ian.M
author_rank: Super Contributor
timestamp: September 22, 2020, 10:54:57 am
content: The gain of a PN200 drops off pretty rapidly over about 30mA Ic. I would suspect that with the limited base drive available from LM3914 LED outputs, depending on the total load you have on each transistor, its not staying in hard saturation, hence the higher voltage drop and dimmer LED strip than expected. Check the PN200 aren't heating up too much as you may be at risk of thermal runaway.

author: scottb721
author_rank: Contributor
timestamp: September 22, 2020, 12:43:26 pm
content: Yes I was surprised by the drop across it. They've been getting a good workout and hasn't showed any heating.I'll build the 2nd one completely on the breadboard first and play with some resistor values.I made the mistake of assuming the base current, enough to drive the original LEDs, would be sufficient to saturate the PN200s.

author: Ian.M
author_rank: Super Contributor
timestamp: September 22, 2020, 01:22:51 pm
content: How many mA Ic do you need for the strip load? You may need to goto a Darlington or Sziklai pair configuration to get enough current gain if you need more than a few tens of mA load current.

author: scottb721
author_rank: Contributor
timestamp: September 22, 2020, 01:39:36 pm
content: Based upon the existing resistors it's 20mA, therefore 60mA per section of 3 LEDS. For the two purple sections I'm driving 2 colours together so would be 120mA.Thanks for the input by the way.

author: Ian.M
author_rank: Super Contributor
timestamp: September 22, 2020, 01:55:09 pm
content: That doesn't leave you much (if any) margin if the LM3914 circuit has been built to the reference designs with only 1mA reference current at pin 7, which only gives about 12mA LED current (which is your base drive). If so, try adding a 1.2K resistor from LM3914 pin 7 to ground to increase the LED drive to approx 25mA.What voltage drop are you measuring across the PN200 driving the purple sections?

author: scottb721
author_rank: Contributor
timestamp: September 24, 2020, 11:15:13 am
content: Bit of a change up.I moving to a Sziklai pair with the PNP hung off VCC and using it to drive an NPN to switch the LED cathode.