hi! sorry for my newbie question. I just started arduino (from the kit), then was up to do a simple project. (alarm)
it was an alarm system (consists of reed switches, PIR motion sensors) that post logs to facebook. things have worked smoothly (and i'm able to do some workaround to prevent fb token from expiring every hour) until the PIR sensors outdoors are triggering false alarms on times that temperature changes. so, im planning to add something like a laser tripwire to verify if it is a false alarm.
I got my laser module and was informed that it runs on 3.7V and at around 100mA. I've read that arduino output pins run at 10mA and should really be careful not to exceed 40mA. It should have been easy switching things higher than 5 v (learned from the motor example on arduino starter kit), so I was thinking if there is something like that I can do. also, I've read somewhere that I can put around 3 output wires together to get somewhere near the max current per pinouts to be able to sustain the current requirement of the laser, but I need around 3.7V for it to work. if possible, I want to avoid having the laser turned on for long time, so I only want it to turn on for some 3-5 minutes when something was detected by PIR sensors, then check if the laser beam will successfully reach the LDR at the end.
...i'm not sure if I can switch it the same way I've switch a larger volt requirement motor, so please advise..
Are you sure that the 100mA rating is for the input and not for the power supply? Look at your specs and verify. I am betting you are confusing the input current rating with the power supply current rating of your module.
...i'm not sure if I can switch it the same way I've switch a larger volt requirement motor, so please advise..
The same kind of transistor switching circuit for like a simple motor, solenoid, relay, etc would work fine for switching your laser module, it work fine even thought you are switching a lower voltage device.
Attached is the datasheet for a common NPN transistor that can be used between the neg term of laser and +5V. It can be switched with a logic level signal from a digital output or a PWM signal ..The Col is the + term of the transistor that connects to laser neg term. The emitter is the neg term of transistor that connects to ground. You need a dropping resistor between +5V and the pos term of laser. The value is given by the formula for a dropping resistor:
Rdrop=(Vin-Vload)/Iload=(5Vdc-3.7V)/0.1A=1.3V/0.1A=13 ohms.. (5-3.7=1.3V)
Power rating for resistor is given by Ohms Law Ohm's law - Wikipedia
PRdrop=Iload x VRdrop=0.1A x 1.3V=0.13W= 130mW. ==>12 ohm ,1/2 Watt
The same kind of transistor switching circuit for like a simple motor, solenoid, relay, etc would work fine for switching your laser module, it work fine even thought you are switching a lower voltage device.
If you want 3.7V from a 5V supply, given that the current is "around 100 mA" from a possibly less than reliable source, the simplest way is as I described, two silicon diodes to drop about 1.2V and allowing for about 100 mV in the saturated transistor you have 3.7V with no compromises.
So - you are not one of those who reminds contributors to size their pictures appropriately then?
So - you are not one of those who reminds contributors to size their pictures appropriately then?
Full screen is appropriate the way I see it. XD
Frankly, I think both our circuits would work. I don't think the dropping resistor is a compromise but it's up to the OP anyway.
I mean it's not like we're competing for karma clicks... is it ? XD
Hi, I think you had better just not have outdoor PIR, if you are going to use a trip wire to prove it isn't a false alarm, why not just use the tripwire on its own.
I have never heard of PIR being use outdoors except for automatic lighting, security indoors yes.
You can tolerate a lamp coming on at times due to birds but if it is a security system I'd be looking for something more reliable.
@Paul__B,
This is what the OP asked for. The circuit I posted fits this description.(but with added dropping resistor)
The same kind of transistor switching circuit for like a simple motor, solenoid, relay, etc would work fine for switching your laser module, it work fine even thought you are switching a lower voltage device.
