Frage Max current ports

How much current can each port handle? 20mA or more?

I want to connect a crosshair laser to Relay Output 2. It's rated 5V and 55mA.

Hi,

The line driver IC's are type SN74HC244 if I remember correctly - the device data sheet specifies an output current of 6mA both sinking (at Vol) and sourcing (at Voh) at a Vcc of 4.5V

Connecting your laser device directly to such an output will greatly exceed these limits and may well damage or destroy the device concerned.

You will have to add a suitably rated buffer that will accept a normal TTL input and provide the current you require for the laser. A single bipolar junction transistor would fill the bill provided you suitably limit the input drive current to say 3mA and protect the output against any inductive transients that might be present when switching.

A 2N3904 would do the job easily with a 1.8K resistor in series between the relay output and transistor base. Protect with a 6.8V zener diode connected collector to emitter. The output will switch on when the relay output goes high (+ve).

Peter

Thank you Peter,

Could you possibly draw a simpel layout of what you described?
I'm not sure where to put the zener and what's it's purpose.

I have 2N2222 transistors here, would they do the job as well?

Hi,

Your 2N2222 is fine for the job - specified at 30V Vce and 800mA Ic.

Zener diode provides protection against inductive spikes - I've suggested a 7.5V device to avoid any chance of conducting at your 5V supply level.

Quick schematic below - use a small piece of veroboard to mount parts and keep it all neat & tidy

Thank you very much! I get it now.
Now I'll wait until my laser gets delivered.