I have a 12V pulse square signal coming from an RPM sensor. I would like to translate it into 5V pulses since I will need to use it as input in an Arduino. What do you guys think is the easiest way? Can a comparator be used?
Arduino – Converting 12V Pulses to 5V Pulses
12v5varduino
Related Solutions
How much current do you require? For specs you want a FET with a breakdown voltage (BVDSS) above 12V and a current rating that exceeds your load. A logic-level FET can sometimes be better but it probably will not matter if your current requirement is only a few amps. N-channel is usually what you want.
I use the On-Semi NTP65N02 with a 3.3V gate drive level for a current-sink. TO-220 package and a fairly low Rds(on) at that level.
Whether a transistor directly across the contact is a good idea or not depends on how the low side of the switch is configured. If that's just an open-drain/open-collector then there's no problem. If it would have a very weak pull-up then the transistor's emitter will be a few volts positive with respect to the base, and though the voltage may not be high enough to damage the transistor, NPN transistors don't like that.
A safe alternative is to use an analog switch,
like the 74LVC1G66, which is an improved single gate version of the good old CD4066 quad switch. The 74LVC1G66 has an on-resistance of less than 10 Ω, but at least a few tens of ohms should be no problem.
Many of these switches have near-zero power supply current (typically 100 nA for the 1G66), and contrary to mechanical relays don't wear out.
If you can't connect the heartbeat monitor's power supply to the keyboard's you'll have to use an SSR (Solid State Relay), like Dave suggests in his answer. These are more expensive though: the Toshiba type Dave mentions costs almost 4 times the 1G66, so I would only use that one if you really need the isolation.
Best Answer
Several methods:
The method you choose depends on how much electrical noise your environment has. If it’s a car or motorcycle engine, this might be a lot.
There are combo solutions such as using a voltage divider and a zener and a transistor or FET to condition the signal. That said, the optoisolator is the most robust solution as it breaks up any potential current loop between the sensor and your Arduino. It’s also more expensive, but much less than the cost of your fried board.
There's an additional benefit to using an optoisolator: by referring the diode input to the sensor ground, it will reject common-mode noise that might find its way onto the sensor wires.
More here: Detect car +12V using Arduino
And a Littelfuse appnote about automotive noise
MORE: Here's a simulation of common-mode noise with the optoisolator (try it here). I added a Zener to protect the opto LED, it's optional.