Raise voltage slightly

Hi,

I am a newby (to electronics).

One of the introductory circuits I am building adds a motion detector function to an existing rechargeable LED work light.

The PIR I am using outputs 3.3V.

The relay (reed relay) I am using has a pickup voltage of 3.5V.

I need to raise the PIR signal from 3.3V to 3.5V or greater.

I have tried to increase the voltage using a step-up booser but it is set to 5V as output and I think the PIR signal does noty provide enough current.

I am currently looking at a joule thief but are not optimistic.

What is a simple approach to "raise the PIR signal from 3.3V to 3.5V or greater"?

These are the specs:

Ultimate Bat Light
Battery
3.7 V, Li-ion, 1800 mAh
Charging Unit
6 V DC, 300 mA

PIR HC-SR501
Voltage: 5V - 20V
Power Consumption: 65 mA
TTL output: 3.3V, 0V
Current Drain <60uA

Reed Relay
CAT. NO.: SY4036
Contact form: SPST normally open
Coil resistance: 500ohms
Pickup Voltage: 3.5V DC
Allowable Voltage: 160%
Power Consumption (W): 0.1
Contact Rating: 1A - 5 VOLT

Thanks,
Nic

You have a 3.7V battery I think?
There is a 6V charger?
The PIR needs 5V to 20V. Where is it getting its power? from the battery?
The relay needs 3.5 to 5V to turn on.

The standard way to drive a relay is to use a transistor. Power for the relay can be from whatever is powering the PIR module (if it's more than the 5V required by the relay, then you can connect a resistor in series with the relay coil).

Why are you using a relay? A transistor or logic-level Mosfet can replace the relay.

Dougy, the transistor base current should be 1/10th its collector current for it to saturate fairly well. But the 10k series base resistor value is too high so the base current is way too low.

#2
> Where is it getting its power? from the battery?
Yes the 3.7V battery

#3
Is the diode a part of the relay circuit or do I add it (the coil looks a part of the relay)

#4
> A transistor or logic-level Mosfet can replace the relay.
Can you point me to an example of this please?
> But the 10k series base resistor value is too high
What size resister do you suggest?

Thanks,
Nic

The diode needs to be added to the relay.

The relay is a reed relay as attached.

Where does the diode connect?

What specs should the diode be?

An IRL540 Mosfet can be turned on by the PIR and it can turn on the LED light.

Thanks audioguru.

I've ordered an IRL540 from an ebay seller; says its local (AUS) stock, so it should be here within the week.

Any pointers on how to wire it?

I do not buy anything from ebay because there are many fake or bad parts there that do not work. I buy parts from a real electronic parts distributor, and their parts are genuine.
A Mosfet needs special handling to prevent static electricity from damaging it. You need to know where to get its datasheet that has a photo showing which pin is which.
Sorry newby (to electronics), maybe someone else can teach you the basics, MATE.

Good point about the ebay purchase. Good point about the datasheet and the pins. Believe it or not, you are teaching me the basics. Thanks.

Message is unavailable.

audioguru said:

Dougy, the transistor base current should be 1/10th its collector current for it to saturate fairly well. But the 10k series base resistor value is too high so the base current is way too low.

Click to expand...

While the schematic I posted was just the first I found in google images, the 10k base resistor will probably be fine (the relay pickup current is only 7mA, and the transistor won't be saturated with that current).

dougy83 said:

While the schematic I posted was just the first I found in google images, the 10k base resistor will probably be fine (the relay pickup current is only 7mA, and the transistor won't be saturated with that current).

Click to expand...

If the coil of the reed relay uses only 7mA from 5V then its resistance is as high as 714 ohms. Its wire must be extremely thin.
Years ago 24V relay coils were 700 ohms.

audioguru said:

If the coil of the reed relay uses only 7mA from 5V then its resistance is as high as 714 ohms. Its wire must be extremely thin.
Years ago 24V relay coils were 700 ohms.

Click to expand...

It's a reed relay, which generally have a higher coil resistance than standard power relays, as you know. The OP said the relay coil resistance was 500 ohms, with a pickup voltage of 3.5V.