Small LED string power supply

Background:

Last year I purchased a small LED lighting strip as a Christmas decoration. It was powered by two 1.5v batteries and whilst the lighting strip gave an effective display I found that the batteries didn't last too long. This year I investigated an alternative 3 volt power source and I also wanted to make use of some spare components I had.

I had a 240 volts (AC) mains power block for providing 5v DC to charge Sony Ericsson mobile phones and using this as the primary power source I investigated a suitable 3 volt regulator. The LP2950-30 provided the solution. I put together the simple circuit below. LED D1 provides an indication that the 5v supply is available. Capacitors C1 and C2 provide stability of operation for the voltage regulator as recommended in the LP2950-30 datasheet.

Specifications:

Input voltage supply 5v @ 450mA Sony Ericsson power block.

Output voltage 3v @ 100mA (max), to supply 50mA for LED string.

5v to 3v voltage converter circuit

Construction :

The circuit components were mounted on a very small piece of veroboard, placed within a small plastic box - Hammond 1551GBK.

To maintain separation of input and output connections, a 2.5mm DC power socket was used for the 5v input supply with a 2.1mm DC power socket used for the 3v output supply. The LED was mounted next to the input socket to act as a reminder that the neighbouring socket is for 5v input.

Operation:

The LED strip connects to the output socket using a 2.1mm DC power plug. The 5v power block connects to the input socket using a 2.5mm DC power plug. A regulated and stable 3v was measured at the output, with current draw of 51mA. Initially running for an hour with the plastic box lid fitted, the regulator was cool to the touch - working well below its maximum current capability. No more batteries will be required! Images of the completed unit are below:

Circuit board mounted within small box

Lights are on, no more batteries!!

Components:

LP2950-30 (3v Regulator)

2.2uF 50v Electrolytic capacitor (2)

D1 - Red LED as main power supply indicator

LED panel mounting socket

R1 - 220 ohms 0.25w

Supply input socket 2.5mm DC socket

Supply input plug 2.5mm DC plug to connect 5v power block

Supply output socket 2.1mm DC socket

Supply output plug 2.1mm DC plug to connect to LED string cable

Plastic project box Hammond 1551GBK 50x35x20mm

Veroboard for circuit mounting

Passive Infra-Red Controlled LED Strip Lighting

Introduction

In my house there is an inglenook fireplace which houses a wood burner.

Ordinarily the fireplace is quite a dark place being recessed and I wanted to provide some lighting to take the darkness away and to light up the natural stone.

After searching for lighting and considering the powering options I decided to install an LED lighting string. These strings come in various sizes and can be cut to length. Powered usually by 12 volt power supply I wanted to hide as much cable and components from view.

Looking at various switching options I decided to design a switching unit based on a Passive Infra-Red (PIR) detector and power transistor to switch the 12 volt LED string on and off.

The main challenges to address are that the PIR detector alone can only switch low power circuits and the PIR output is limited to a short 'on' time once activated. What was required is that the PIR detector should operate with a single hand gesture with circuitry to switch the LED string on, and with another hand gesture, the PIR switches the LED string off.  The PIR module will be mounted out of sight underneath the inside of the fireplace recess, so a single hand wave will be sufficient for the module to detect movement. One wave will switch the LED string on, with another wave switching the LED string off.

The circuit below achieves what I required.

Passive Infra-Red LED string switching controller

Circuit description

The PIR module detection sensitivity is set so that it is triggered when movement is as close as possible to avoid spurious activity. The 'on-time' is set to its shortest time, as all that is required is a single pulse output from detected movement.

The PIR module output is fed to a CMOS dual D-Type Flip-Flop. Only one Flip-Flop of the CD4013 is needed in this example. The Flip-Flop is configured so that when the PIR output changes from Low to High, the output of the CD4013 (Q) goes High. When the PIR output returns to Low condition, the Q output of the Flip-Flop remains High. Only when another transition from Low to High on the PIR output takes place will the Flip-Flop change its output from High to Low. In effect the CD4013 Flip-Flop is performing a divide by 2 operation, but in this case its output can be used effectively as a latch.

The output from the CD4013 is fed to Q1, MOSFET  RFP12N10L. The MOSFET is switching 12volts to the LED string, and with the length of string I needed, the total current draw was approximately 0.5 Amp. The MOSFET is working well within the operating specifications and does not experience any rise in temperature, which is important as I chose to mount the circuitry in a sealed aluminium box.

I included the LED (D1) as a switching indicator. Should the LED string fail to operate and D1 is on, this will indicate that the LED string has failed.

The PIR module and CD4013 require a 5v supply. The power regulator 78L05 provides this and is capable of supplying up to 100mA.

Pins 8, 9, 10 & 11 of the CD4013 are taken to 0v. [not shown on the circuit diagram above]

Operating currents

In operation, this circuit draws a relatively small amount of power; indeed when inactive, current draw is almost zero.

The following total current measurements were taken:

PIR inactive: 51uA

PIR active: 0.375mA

CD4013 inactive: 0

CD4013 active: 41uA

LED strip off: 51uA

LED strip on (3 metre length): 400mA

LED1 on draws 1.6mA

References / data sheets

HC-SR501 PIR module

CD4013 dual D-type Flip-Flop

78L05 5v power regulator

MOSFET  RFP12N10L