SparkFun Forums

[mark_anderson_us]

Hi All

I'm pretty new to electronics. I'm building a circuit where I need to sense a DC signal between 9 and 24v DC and an output to a microcontroller. I want to use a PS2501-4 (http://www.cel.com/pdf/datasheets/ps2501.pdf), but I'm kind of stuck on figuring out the range of input voltages it supports and what I should use to ensure this range of voltages will trigger it.

I looked at the data sheet, but can't for the life of me figure out what the max input voltage is.

In addition to this, I then need to figure out how to generate enough forward (for the 9-24v input) to trigger the device. I'd like to keep the no of components to a minimum due to size limitations.

Any help/explanations would be appreciated

Thanks


Mark

[hneve]

Diode char. 1.17v at 10mA .
Its all in the sheet

[mark_anderson_us]

Unfortunately, I don't understand that, I took that to meant that the forward voltage drop across the component is 1.7v, so if I have 10v at anode, i'll have 8.3v at cathode.

[waltr]

The diode is an LED. So use the same calculations for the resistor (series current limiter) to light an LED.
It is the LED current that you set.
A 9 to 24V input range is quite large. What exactly do you need this opto to do?
Have a logic low when the input is about 9V and a logic high when the input is about 24V?

[mark_anderson_us]

The diode is an LED. So use the same calculations for the resistor (series current limiter) to light an LED.
It is the LED current that you set.
A 9 to 24V input range is quite large. What exactly do you need this opto to do?
Have a logic low when the input is about 9V and a logic high when the input is about 24V?

Thanks Waltr

I just need to get a logic low whenever there is a signal on the input, which could be anywhere from 9v to 24v. I'm being very conservative here. I'm predicting that 99% of the time (possibly 100%) it will be 10-15v), but don;t want anything thing blow up if there's a 24v signal (it's an automotive application).

So I need to find a way to limit the signal to 9v (or lower) and then drop it down to 1.17v

Regards

Mark

[Mee_n_Mac]

It seems feasible at 1'st glance. If you aimed for a min of 10 mA with 9v, you might pick a resistor near 760 ohms.
(9v - 1.4v)/760 = 10 mA
At 24v the current might be as high as 30 mA, which is under the 80 mA spec limit.
(24v - 1.1v)/760 = 30 mA

One thing I didn't check was power dissipation, per channel and as a whole.

If you restrict the transistor to sink 1-2 mA, you should get < 0.2v btw collector and emitter (the latter presumably tied to "ground").

Is there some max "no signal" level, that might be on the line due to noise or coupling, that the circuit is not supposed to declare as *signal present*, even if for just a short time ? ie - a false alarm rate or signal level ?

[waltr]

Since this is for an automotive use, I would add protection from spikes to ensure the diode doesn't get fried.
A simple way is to split the series resistor into two series resistors and put a 12V TVS from between the resistor to the ground.
Do search for 'protection circuit for automotive use' to learn more.

[mark_anderson_us]

Thanks Guys!