Delorean project

JoeJester

Joined Apr 26, 2005
4,390
I'm sorry if i'm offending anyone, but I don't want to put my whole circuit up on here.
You can design in blocks. Specific inputs and desired outputs, stage by stage.

This forum becomes your Engineering Notebook per se', as you have your notes reviewed by someone knowledgeable in the subject.
 

Thread Starter

Rissy

Joined Nov 23, 2015
106
Cheers, chaps, glad you understand.

So my circuit basically consists of a bunch of NAND's, carrying out some logic based on a number of inputs, each of which is taken from different areas of the vehicle wiring, so each is a variable 12-14V which can either be on or off. Depending on the on-off status, my logic turns on and off an automotive LED (it's hoped, i've not got that far yet)

I'm currently trying to sort out the dirty characteristics of the vehicle 12V system, to make these inputs clean for the chips. So this would be described very much as the power and input handling of the circuit.
The logic component, i've taken care of now, and i'm happy with.
For the output, straight off one of the NAND gates, i've got a 10K resistor, followed by an BC548B NPN transistor. The measurements on the simulator (Not taken them for real) are: (to the base of the transistor) voltage = 700mV and from memory, a very small measurement of uA (I'd have to check again after work) The current through the LED looks to be 8mA with a leading 10K resistor in the simulator. In real life, i've removed that resistor because the LED was too dim and worked fine without the resistor, so i'm sure the current will be much more with this resistor removed, but i've not measured it)
All signs from the simulator are looking good, and in real life, using a breadboard, i've got the logic working, turning on and off a 12V LED without problem. Obviously to do this, i'm physically switching wires from positive to negative. I say without problem, but i've also been experimenting with potential dividers, to check to make sure the transistor still switches the 12V through the LED even though the rest of the circuit is running at a lower voltage (9V or thereabouts), and i've found that while i'm changing state of the inputs between ON (9V) and OFF (0V), the LED on the transistor is staying on, all be it dimmer than fully ON state, and doesn't go out fully until that input is properly plugged into 0V!? It didn't do this until I introduced a potential divider (only one, which i'm using to switch ALL inputs between ON and OFF). I'm not sure if this is a result of the sharing of the same 9V supply for the inputs (I only have a small breadboard) or not, or if it is due to current leakage through the potential divider on to the 0V line, which EVERYTHING is sharing at all points.

As I said before, i've simulated a L7809CV for the voltage regulation from the 12V -14V down to 9V and the results look good. I've ordered some of these from Ebay, and have intention on sticking one of these (with 0.33uF and 0.1uF capacitors as advised in the datasheet) on each of the 12V inputs, as well as VDD too. I'm hoping this will be ok. I've read about people using these regulators to successfully pass switched signals, and there is no problem unless you go higher than 1MHz on the switching through them. This will not be a problem for me, as I'm going to be switching closer to 1Hz than anything (only for 1 signal - others will be constantly on or off depending on the state of other controls on the car.)

Does this give everyone enough to go on? (except the definite values of measurement in real life and/or the readings from LTSpice, which I'd have to get for you tonight some time)
 
Last edited:

Thread Starter

Rissy

Joined Nov 23, 2015
106
Then at least answer my base question? What do you have for base resistance on the transistor? And what is the current drawn by your 12V LEDs?
You can read my other, much longer message, but basically i'm using 10K resistance on the base, and i'm looking at 8mA of current through the LED.

*EDIT* Ignore all this, I was working on bad memory! LOL!
 
Last edited:

Thread Starter

Rissy

Joined Nov 23, 2015
106
...Just to update, i've finally got on to LT Spice again, and i'm reading 400uA on the base of the the BC548B NPN transistor, along with 800mV.

Through the output LED which is driven by the transistor, i'm getting 56-57mA with direct 14V applied to it (I've removed the 10k resistor I had in there originally.)
 

Thread Starter

Rissy

Joined Nov 23, 2015
106
I don't have a datasheet unfortunately, but it doesn't seem to be unhappy right now (I only have 12V applied to it in real like since I've got a 12V power supply, so I'm sure the real world amperage is lower than LTSpice measured with 14V)

I think BA9S LEDs can handle something more like 125mA or so, so I think once I hook one of those up, it'll be okay.
 
Top