Oscillator & Constant Current Driver Dislikes Playing Together

Hi all, i have created a circuit attached below that uses the analysir constant current driver for a single TSAL6100 IR LED and a NE555 square wave oscillator module from ebay.

The oscillator circuit uses NPN transistors to allow the oscillator to be modulated by an arduino, the purpose is to remove the internal carrier frequency processing from the Arduino and thus make it simpler to code, reduce strain on the microcontroller and avoid making the timing anymore complex.

My issue is that if i connect my USB oscilloscope in parallel with the infrared LED i blow the led, if i measure using my multi meter to check frequency i get 55.79khz directly from the oscillator module after the transistor i get 289hertz and when i put my multimeter in series with the infrared led i get 50 hertz and 67 millamps as appose to my 200 millamps as appose to 114 millamps if i remove the extra transistor and oscillator module.

Anyone any ideas what i am doing wrong, i am not the best at building electronic circuits and find programming them much easier.

Q1 and U1 are backwards in the schematic.

Q2 doesn’t have a base pull-down resistor, so it may switch off slowly.

I worry that 1k resistor isn’t providing enough base current for Q2 with the low voltages
available (ie its going to have a low Vce and just somewhere closer to saturation than
linear region? in that case derate the min gain from 100 to 50 or so)

Replace the 1k by 220 ohms, add another 220 ohms from base to emitter of Q2.
Q2 does have reasonable PCB area to dissipate heat into?

MarkT: Q2 doesn't have a base pull-down resistor, so it may switch off slowly.

I worry that 1k resistor isn't providing enough base current for Q2 with the low voltages available (ie its going to have a low Vce and just somewhere closer to saturation than linear region? in that case derate the min gain from 100 to 50 or so)

Replace the 1k by 220 ohms, add another 220 ohms from base to emitter of Q2. Q2 does have reasonable PCB area to dissipate heat into?

Hello, I am trying to learn more about circuitry. I understand how a transistor works, but what i the purpose of having one transistor control the base of another transistor? Thanks

the purpose of having one transistor control the base of another transistor?

Either on/off control, or additional current gain.

Yes, i realize i made an oopsy on the schematic regarding the LED and Q1, i also left out by mistake the 330 ohm resistor on the base of the transistor i was previously using a 1k and dropped to 330 which appears to provide me with a matching frequency on the output.

My issue now is that i cant seem to reliably measure the frequency at the output of the constant current driver if i use my multimeter to measure it i get a fluctuating value that isn't really possible to measure, i applied 5 volts to my modulation input pin so i should just get the carrier frequency of 56khz through.

All this being said i just deconstructed the circuit and rebuilt checking resistor values and placements now i get 56 khz on the output of my infrared LED so must of been a bad placement or a moment of madness somewhere.

My only issue now is that using the two circuits together i have got a slightly lower current output of 78 millamps as appose to the 110 millamps i was getting before (website states i should get 200 millamps but 100 millamps due to modulation but when i drive it without any modulation i still only get 100 millamps and my additional transistor on the opposing side seems to drop it down to 78.

The reason i am using a two transistors to drive an LED is that i have a square wave signal coming out of the 555 timer module that i need to modulate with an output from an arduino, look infrared signal diagrams to understand but dont want to accomplish this by directly turning on and off the whole 555 timer every 600 microseconds or so.

Hope ive explained everything well enough, and atleast for the most part the infrared operates correctly even if the current is a tad low.