# Hacked pulsed laser driver using Arduino

I replaced the laser diode with a resistor, and the battery with a lab PSU set at 3V.
I am still getting a flatline of voltage across the resistor. The PSU reports no current flowing through it.
Think all my experimentation has killed the transistor?

#### Attachments

• 10 KB Views: 33
• 21.2 KB Views: 38
BvU
Homework Helper
2019 Award
the battery
What battery ? Don't see none...

Paul Colby
Gold Member
Okay, one needs a battery. $V_{BE}$ should be about +0.7V for a silicon NPN for it to be saturated. $V_{CE}$ in this state would be zero should the battery be absent. Your scope trace shows a negative $V_{BE}$. Signs of voltages and currents really matter here. An NPN is like a current control where a small current, $I_B$, injected -into- the base results in an $I_{E}$ $\approx \beta I_{B}$ where $\beta\approx 100$. I think the transistor (any NPN transistor) spec sheet would have all these things plotted and would be helpful for you.

OK guys, got it working
Thanks for everyone's help.
The problem was the transistor - it was faulty.
Replacing it with another transistor got the circuit to work.
The circuit without the battery with a PSU instead is attached below.

#### Attachments

• 20.7 KB Views: 32
• 11 KB Views: 31
Last edited:
BvU
Homework Helper
2019 Award
Looks as if it only works because the transistor is abused to short-circuit the "voltage" source (the psu)

OK, how would you improve it?

rbelli1
Gold Member
How to use a transistor to drive an LED

Your circuit only works because the series impedance of the battery is high. If you put that on a strong enough battery you would instantly explode the transistor.

BoB

Thanks for the article. It is very readable and explains basic concepts which I have forgotten from my mammoth hunting classes.

Last edited: