Project of the Day – Digital Photo Frame Low Voltage Disconnect

Posted on April 30, 2013 by neil

Today we put together a special order low voltage disconnect (LVD) to be used with a product display case containing a digital photo frame.

To make the case portable, instead of using the standard 5VDC power supply, 8 series connected high capacity C sized rechargeable cells have been fitted to give a 9.6V power supply which goes through a dc-dc voltage converter to get the required 5VDC to the photo frame.

Low voltage disconnect for digital photo frame

Unfortunately when the voltage of the 8xC battery pack falls to 8.7V (when the battery pack is almost empty of charge), the photo frame software crashes and gets stuck in a constant reboot cycle due to not having enough power to restart.

The low voltage disconnect we designed measures the voltage of the battery pack and if is more than 8.80V it outputs power to the voltage converter and photo frame. If the measured voltage falls below 8.80V, then the output is turned off and only turns back on again when the measured voltage is found to be greater than 9.20V.

The display case is fitted with a standard off the shelf battery charger which takes a 12VDC input. An external 12VDC power supply can be plugged into the case to recharge the battery pack while at the same time powering the photo frame via the low voltage disconnect circuit causing the disconnect to be cancelled (since the measured voltage is now 12V) and the photo frame to restart.

When the 12VDC is supplied, the 8xC cells are automatically disconnected from their series circuit and connected individually in parallel to the battery charger for charging. When charging is complete, the 12VDC power supply can be disconnected, the battery pack voltage will be >9.20V, and so the frame will continue unaffected.

We sell our own automatic user programmable low voltage disconnect in the REUK Shop. If you have any special requirements which this product does not meet, let us know and we should be able to put something together for you.

Project of the Day – 12V PIR Alarm System

Posted on April 27, 2013 by neil

Today we put together a 12V alarm system for a farm building located 100 metres from the farmer’s home. When motion is detected in the building by one or two standard , two relays close. The first causes a loud siren to sound, and the second turns on a bright strobe light visible from the farmer’s home.

If the controller is not reset by the farmer pressing an external panel mountable button then after 3 minutes the siren relay opens cutting the sound, but the strobe light is left turned on until the controller is manually reset to ensure that motion detection events are never missed.

If you need something along these lines then let us know. Click here for suitable low power consumption 12V strobe lights and/or low power high volume 12V sirens and sounders for your own projects.

12V Regulator for RC Planes, Helicopters, and Cars with Cameras

Posted on April 26, 2013 by neil

With the price of high quality ruggedised mini video cameras and transmitting equipment falling rapidly, many people are now attaching FPV (first person view) cameras to their radio controlled planes, helicopters, and cars with very impressive results.

One common problem though is interference. When radio transmitting equipment is powered by the same power source as motors, servos etc, interference (noise) can add lines and other unwanted effects to the transmitted images.

The camera and transmitter need a very stable fixed voltage – e.g. 12.0 Volts – as anything else will damage the sensitive electronics. Therefore a voltage regulator is required. For their high efficiency and small dimensions a switching type regulator would appear to be the obvious choice, but this type of regulator generates yet more interference. Therefore a lower efficiency linear regulator must be used.

In the second half of this excellent article The Tricopter V2.6HV David Windestål from Sweden explains in detail how he built a very stable interference suppressing linear regulator around the LM2940 low dropout voltage regulator.

This is almost identical to our standard REUK 12V regulator but the components L1 (1mH coil) and C1 (low equivalent series resistance ESR also known as low impedance 22uF capacitor) have been added working together to make an LC filter (Wikipedia: electronic filter) to strip out any noise from the ESC (electronic speed controller) and everything else connected to the same battery pack.

Regulator for RC helicopter camera transmitter

With a 16.8V LiPo battery pack fitted in his DIY tricopter and a transmitter drawing 300mA, the regulator only has to get rid of 1.5W of heat, so only a small heatsink was required resulting in the finished regulator ended up small and light which transmits video which is “crystal clear [without] a hint of interference”.

Testing Car Battery Voltage Meter

Posted on April 19, 2013 by neil

Pictured above is a car battery voltage meter which is designed to be inserted into the cigarette lighter socket of a vehicle to test and display the battery voltage – purchased for £1.76 including delivery from Hong Kong.

It is relatively well made if a bit plasticky, but certainly as good as should be expected for a device at this sort of price. Inserting it into the car cigarette lighter socket it displayed the battery voltage accurately to well within 0.1V, and when tested with a variable power supply gave similarly reliable readings with an input voltage from 6V to 16V.

The LED numerical display is bright – if anything too bright as the voltmeter draws 40mA when it is on which is too high to leave connected to a battery all the time (taking 1Ah of charge from the battery per day).

We purchased this to test and to take apart for a project. Breaking it open was very easy.

The plastic end cap is threaded onto the positive end of the cigarette lighter plug, and can be unscrewed (with a bit of force). Inside is a 5A fuse which should really be a 100mA fuse to properly protect the device.

Then the two sections of black plastic can then be prised apart to expose the innards.

There is a small circuit board with an IC on it, the LED display, a large resistor, some other common components, and a TL431 adjustable precision zener shunt regulator which is used as the voltage reference.

We wanted this to stick on top of a solar charged 12V lead acid battery to display the battery voltage so we chopped off all the unnecessary bits and pieces leaving just the two wires for connection to the + and – terminals of the battery.

The blue plastic cover is pretty much vital as without it it is very difficult to read the voltage from the display.

Finally, to reduce power consumption, we wired a small push to make button to the white positive input cable and connected that and the black negative cable to the terminals of the battery. Now, whenever the button is pressed and held, the battery voltage is displayed. This mini LED battery voltmeter is only 3.5 x 2.5 x 1.0 centimetres in size and does the job perfectly.

If you are interested in buying one of these car cigarette lighter LED voltmeters click here.

L298N Dual H Bridge Motor Drive Controller

Posted on April 8, 2013 by neil

An H Bridge is a electronic circuit which has an output voltage which can be in either direction. This is particularly useful when used with motors as reversing the polarity of the power supply to a motor causes it to rotate in the opposite direction.

In our article Automatic Hen House Door Controller we showed how a pair of relays could be used to achieve this polarity reversal for motors, as making an H bridge can be a bit complicated and mistakes tend to lead to destructive short circuits. Fortunately now there are many ready made H bridge boards available relatively cheaply which make everything a lot simpler for the DIYer.

Pictured above is one such H Bridge circuit. Like most it is actually a dual / double H Bridge and so it can be used to control the direction of motion (and on/off status) of one or two motors.

The row of three screw in-terminals pictured at the bottom of the image are for the +12V and GND input connections, and a +5V output connection respectively. The +5V comes from an onboard 5V regulator which can be used to directly power a microcontroller such as an Arduino or Picaxe which would usually be used to control the motor(s).

The pair of terminals on the left of the image connect to the two power inputs of the first DC motor, and the second pair of terminal on the right to the second motor (if used).

There are 4 yellow inputs on the bottom right of the board as shown in the image labelled IN1, IN2, IN3, and IN4. If a high logic signal is sent (by the microcontroller) to IN1, the first motor will turn in one direction. If a high logic signal is sent to IN2 then the first motor will turn in the other direction. IN3 and IN4 work in the same way with the second motor.

It is not actually necessary to use a microcontroller – microswitches and resistors can be used to control the motor(s) in some simple applications.

This H Bridge circuit is based around the L298N from ST Microelectronics – a dual full bridge driver which can supply up to 2 Amps of output current to each motor.

Click here to purchase one of these: Dual H Bridge Boards. Prices start from just £2.50 including delivery (air mail from China – 3 weeks) to up to £10 from UK sellers. As it typically costs around £5.00 to purchase just the L298N chip from electronics suppliers in the UK, these boards are great value and speed up project development no end.

Testing LM2596 Variable Voltage Regulator

Posted on April 8, 2013 by neil

Our LM2596 Variable Voltage Regulator (discussed previously here: regulator with display) has finally arrived after its long journey from China.

The first image above shows the input voltage as being 13.8V (which was accurate to <0.1V). The second image below shows the output voltage which we set by adjusting the small brass screw in the blue potentiometer to be 5.0V (which was also accurate to <0.1V).

lm2596 with LCD voltage display

The build quality and quality of the components used is excellent throughout, the LCD is bright, and the addition of a red LED and a green LED to indicate whether the input or output voltage is being displayed is very useful. Selecting whether to display input or output voltage is achieved using a small button on the regulator board. In addition to screw-in terminals for connection of the input and output wires, there are also holes with solder pads in both fine and medium sizes so that different sizes of wire can be more securely attached if required.

The only bad point would be that there is no option to turn off the LCD when it is not needed – i.e. it needs three options (display input voltage, display output voltage, or display nothing) instead of the two it has. The LCD draws approximately 25mA all the time, so will take 0.6Ah of charge out of a battery every 24hrs which is a lot. Another 10mA seems to be the quiescent current lost in the regulator when nothing is connected to the output but it is still regulating voltage.

We can think of many uses for these voltage regulators which are made easier thanks to the on board display – first and foremost for small battery charging from a 12V battery, and/or powering USB charged devices (using a 5.0V output). For battery charging of say four series connected NiMH rechargeables, you would just set the output voltage to say 5.8 or 5.9V and connect it to the battery pack with the correct polarity.

Apart from having slightly high self-power consumption, these regulators are excellent quality and fantastic value.

If you are interested in purchasing one of these regulators, click here: LM2596 Voltage Regulators. Prices are around £3.50 each or £17 for five including air mail delivery from China.

9V from 12V Regulator Module – Alternative to L7809

Posted on April 1, 2013 by neil

About six months ago a Chinese company on eBay had a special offer selling the item pictured below for US$2 each. We bought five and put them somewhere safe and then forgot what they were for.

A quick test today showed them to be efficient 9V from 12V regulators working much the same as the common L7809 regulator.

With a bit of searching around we finally found the details for this product. It is an high current voltage regulator which takes an input voltage of from 12-23 VDC and outputs 9 VDC. No heat sinking is required up to a constant 2.5 Amps of output current, and they are rated to supply brief peaks of up to 4 Amps.

The pin connections (the three legs) have 2.54 mm (0.1 inch) pitch exactly the same as for the L7809 regulator, and the device is also of similar overall dimensions. Therefore, this little PCB can be used to substitute an L7809 (pictured below) where more current than the 1-1.5 Amps (with heat sink) maximum of the L7809 is required.

It could for example also be used instead of the LM317T we use in our standard 9V from 12V regulator (though we have designed that with R1 = 330, R2 = 2K8 so that we get an output of around 8.8V compared to the 9.3V this regulator module outputs – a lower voltage to reduce power consumption for renewable energy powered applications). With thicker cables for the connections this would give a 2.5 Amp constant current 9V from 12V regulator with no need for heat sinking.

This module has a small integrated circuit labelled MP2307DN which a quick search on Google shows up as a 3A constant load current DC-DC step-down power supply module and a monolithic synchronous buck regulator, which has been used in this case to provide a 9V output.

At the time of writing we can only find this device here: L7809 Alternative Module priced at US$4.99 plus US$1.99 for air mail delivery. (The same vendor also has similar L7812 and L7833 alternative modules for 12V and 3.3V outputs respectively.)

Project PCB Boards for 18-pin PIC Microcontrollers

Posted on April 1, 2013 by neil

We use microcontrollers every day for many of our projects, mostly 8-pin but quite often 18-pin for the more complex products we are requested to design and build. We have found the following products to be particularly helpful for prototyping and for one-off projects – 18-pin project PCBs from UK company RK Education.

Pictured above is a RKP18HP board designed for use with 18-pin microcontroller such as PICAXE. This board gives 5 inputs, and 8 TIP121 power transistor outputs for high power applications. It is supplied with a software download socket to get your code onto your microcontroller.

Below is the RKP18Relay8 board which is again designed for 18-pin microcontrollers and takes up to 5 inputs and controls 8 SPDT relays each with LED indicators to show then they are energised. Again the software download socket is provided.

These items are available as just boards (PCB) or as full kits and are very competitively priced – transistor board £1.19, kit £3.49; relay board £2.00, kit £10.99. The kit prices include all the components shown – e.g. all the relays, screw in terminals, LEDs, resistors, etc, so all you need to add is your choice of 18-pin microcontroller and you need some solder and a soldering iron.

The power transistor board takes about 10-15 minutes to solder together, and the relay board around 15-20 minutes. Full instructions are provided and the PCBs are well labelled so it is difficult to make any mistakes.

Many other boards are also available for low power applications, smaller and larger microcontrollers, and much more. For a one off 28-pin or 40-pin microcontroller project (e.g. PICAXE 40X2 as pictured above) the corresponding boards with large prototyping areas are particularly useful.

Take a look at the RK Education website here.

Project of the Day – 24V Low Voltage Disconnect

Posted on March 23, 2013 by neil

Pictured below is one of our standard programmable 12V low voltage disconnect (LVD) circuits which has been modified for 24V battery systems.

The base circuit board used is one we designed and have made for us which is a multi-use board with a high quality matching enclosure. By soldering a few jumper links on the back of the board we can change its functionality, inputs, and outputs so that it meets the requirements of whatever project we are working on. We designed the board with a small prototyping area in the middle to which additional components can be added if required.

In order to use this board for a 24 Volt low voltage disconnect things were a little tricky as all on board components are designed for 12 Volts. Therefore we first added an LM317T voltage regulator (using R1=330R, R2=2K8 resistor) and screw in terminals for the 24V battery to be connected to. The 11.8V outputted from the regulator was then connected on the underside of the board to the usual 12V input terminals which are now not used.

We then added a voltage divider to reduce the approx 24V input voltage sufficiently so that it can be measured using the analog to digital converter (ADC) on the PICAXE-08M2 microcontroller – we needed less than 5V across the full range of input voltages from the 24V system (e.g. up to at least 30V), and wanted to set this up so that the digital value given when 24.00V is connected to this new LVD is the same as that given when 12.00V is connected to the standard LVD.

The ADC converts the voltage it is seeing into a 10-bit digital value from 0-1023 where 0 will be the value if the voltage is zero, and 1023 will be the value if the voltage is equal to the supply voltage of the microcontroller (5.00V in this case).

With the accurate 470K/100K voltage divider we use on the standard 12V LVD we get 35.922 per Volt – i.e. input 12.00V to the circuit, the voltage divider will reduce this before it gets to the ADC (to 2.553V) and the ADC will output 12.00*35.922 = 431. We have hard-coded the 35.922 multiplication factor into our 12V LVD and everything else is worked out from it.

We hand-calibrated the new voltage divider (because the resistors we used in it only have a 10% tolerance). We simply inputted a 24.00V voltage and measured the voltage seen on the ADC pin with an accurate multimeter. DIviding this voltage by 5.00V and multiplying by 1023 to get the ADC value we got 398, less than the 431 it would have been if the resistors were high tolerance. Dividing 398 by 12V we found that the multiplication factor for our 24V LVD was 33.135 per Volt, so we simply changed that one value in the microcontroller code and everything worked perfectly – the only difference is that the user can set the cut in and cut out voltages in 0.2V steps now instead of 0.1V steps.

Project of the Day – Warning Watch Timer for Solo Sailor

Posted on March 22, 2013 by neil

Today we made the yacht watch timer pictured below. It was ordered by a solo sailor to use to ensure that she does not fall asleep for too long while on watch (checking the horizon for potential dangers).

As this is such a vital piece of equipment, we actually put two independent timers onto the same circuit board doubling up on all components including the warning buzzers. Therefore if a component on one of the timers fails, the second timer will continue to work perfectly.

The user can set the number of minutes the timers are to run before the warning piezo buzzers sound – e.g. 5, 10, 15, 20..etc minutes. Timer-2 is then automatically set to run for one minute longer than Timer-1.

While the device is connected to the 12VDC system on board, a pair of panel mounted LEDs flash once every second to show that the timer is running. Each time the sailor checks the horizon they press a panel mounted button to restart the timer countdown.

If the sailor falls asleep or forgets to keep watch then when the timer has run down Timer-1’s buzzer will sound for one minute. (We left the protective sticker over this buzzer to keep the sound level down.) If the sailor still does not press the button during that minute, then Timer-2’s buzzer will start to sound very loud and continue to sound until the button is pressed.

The sailor will fit an ON/OFF switch before the timer device. Each time it is switched on, both LEDs will light up and the two buzzers will sound for one second to confirm that everything is working as it should. The programmed number of minutes for the timer are stored in memory and are not lost when power to the device is disconnected.

The timers are built using Picaxe-08M2 microcontroller chips each powered via its own 5V regulator. The device is fitted with reverse polarity protection.

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