Transistor Needed

And not enough base current.

Are you trying to destroy that optoswitch LED by running it without a series resistor? Poor thing, being abused this way... No wonder the Vce voltage cannot get that low.

Arie

Are you trying to destroy that optoswitch LED by running it without a series resistor? Poor thing, being abused this way... No wonder the Vce voltage cannot get that low.

Arie

That is more than the low power TLC555 can source. For the BC817, a 3K resistor results in a collector current over 150mA, which is more than needed. Of course, an open loop design is a very poor one, dependant upon the actual gain of the transistor, and it is true the collector is not fully saturated. This was never to be the final design. I alleviated the issue of the small current carrying capabilities of the TLC555 by adding an open drain comparator, which also sharpens the signal and helps eliminate an irregular waveform. Adding a small emitter resistor helps regulate the current. Note the C/E-On voltage in the sim is 456mv. This allows for just a bit of regulation using the BC817, so the collector current should be maintained close to the target of 120mA. I will continue to try a few others.

Since it can handle a peak current of 6 Amps and more than 5V, I don't think it will be a problem. The circuit has a duty cycle of about 0.5%, so the sensor could easily handle a full 3.3V applied in 45 microsecond pulses, 120 pulses per second. More to the point, this was a test design whose purpose was to find the most suitable driver for the sensor. If you will look at the "final" design, you will see there is a 3 ohm regulator resistor in there, which will maintain the pulse current below 120mA.

You are trying tgo teach me Ohms Law? That's funny. There is a very good chance I learned Ohms Law decades before you were born.

Nope. 'Just testing near to the edge of the design limits for various transistors.

What about it?

On a sunny day (Sun, 21 Mar 2021 04:42:57 -0700 (PDT)) it happened " snipped-for-privacy@gmail.com" snipped-for-privacy@gmail.com wrote in snipped-for-privacy@googlegroups.com:

Well yes, as you do not seem to know about it.

Glad you like it :-)

If that is the same sort of chance logic you use to design your circuits it explains a lot.

Absolutely.

When asking a question here it helps save time if you actually post a circuit.

The TCRT5000 datasheet states an absolute max rating 60mA for the LED:

Where do you read the max 6A 5V?

Arie

When you are exploring the many ways to control LED current don't forget the tried and tested vbe based current limit...

Of course it does add a little complexity to your pulser.

piglet

'Sorry, it is 3A @ 15 microseconds, not 6 - I mis-remembered - but that is still far above the 120mA @ 45 microseconds surge to which I will subject it. For the voltage at 3A, extend the curve. It's around 5V

That is just about exactly what I am doing. I've replaced the lower BJT with an open drain comparator.

<sigh> I have been an engineer for 38 years, and an avid electronics hobbyist for 10 years before that.

<sigh> I have been an engineer for 38 years, and an avid electronics hobbyist for 10 years before that. My undergraduate studies were also in physics, which means not only do I know Ohms Law, I know the underlying principles which give rise to an emf when charges flow through various media.

It means that I have one helluva lot of experience with electronics.

Again, nope. Honestly, there are not a whole lot of ways to connect a a resistor, an NPN transistor, and an LED.

When critiquing someone's post, it helps to actually read it. I posted both the (trivial) test circuit and the actual circuit design.

Oh, and just by the way, since you don't seem to know it: Ohms Law does not apply to semiconductors (hence the name: SEMIconductors). For a short, ideal ordinary metal-like conductor, the derivative dV/dI of the applied emf relative to the induced current is fairly constant across a modest range of fixed currents for a given temperature, with a moderately weak dependence on temperature. The current induced in semiconductors is not anywhere linear with voltage, although it approaches so in certain conduction regions. For a semiconductor, dV/dI is a curve. In particular for a transistor, it is a 4 dimensional curve, with the induced current being a function of base current, collector-emitter voltage, and temperature.