# Stereotypical beginners electronics project pt2

I’ve got some LCM7215’s on order for a work project so I’ve got some downtime until they are due to arrive. I’m going to run through the LTSpice simulation process, and look at the alternative solutions available.

Assuming you haven’t already, you will need to download LTSpice. This is a free Spice simulator provided by Linear Technology. It’s pretty basic, but it has almost everything I needed to get off the ground. By using the following equations I managed to select starting values for my oscillator.

Using R = 255k, R1 = 1M, R2 = 255k, and C = 10u, we have a T of just over 11 seconds. Interestingly, when I simulated the circuit, my T was around 4.4 seconds. I am using fairly large values as I want to limit the current. The output is fairly constant, but there are a number of problems with my current circuit.

Firstly, I want  to use a single supply op-amp. This means I need to create a virtual ground or else the op-amp will be driven into an undefined state. The simplest way of acheiving this is my tying the inverting input to Vdd through a resistor equal to R1. Next, my output has a duty cycle of 50%. This is a problem as I want my device to be as low powered as possible. A reduction from 50% to 2% duty cycle would reduce my current consumption by a factor of 12. That is the battery would last 12 times longer. To adjust the duty cycle you need to include a diode in parallel withthe negative feedback loop. The idea is that the diode will all the capacitor to either charge or discharge quicker, depending on the polarity of the diode. As we want the capacitor to charge quicker we need to allow the current to flow to the capacitor quicker. Lastly, I have specced out the op-amp that I want to use. The LMC7215 is a low power single supply rail-to-rail comparitor that ticks all of my boxes. The only problem was that it is a National Semiconductor part, and LTSpice is Linear Technologies. This means that the part won’t already be contained in one of the built-in libraries.

To overcome this, National Semiconductor have released the SPICE definition for that part, and LTSpice allows you to include a SPICE directive. There are a number of options when it comes to using custom SPICE definitions, but I will only cover the one I used. I had already built up the circuit around a different op-amp, so to continue I needed to change the model. There isn’t a way to change the model, so instead you have to delete the current one and drop in the replacement. My replacement was based on the “opamp2” model. Once the base model is inserted you need to insert the SPICE directive. This is a button on the toolbar that opens a text box. Take your downloaded SPICE definition and copy the tesxt directly into the text box. Select “SPICE directive” from the radio buttons, and click OK. Next, right-click “opamp2” and type “LMC7215/NS” into the Value field. This is based on the SPICE line starting with “.SUBCKT” comment. Next, we need to tie together pin names and definitions. This is done by editing the “opamp2” symbol. When you right-click on a pin, a box opens allowing you to change the “Label” and the “Netlist order”. The “Label” is the pins name and the “Netlist order” is the order at which they are mentioned in the SPICE file. In this example we have

“.SUBCKT LMC7215/NS     3  2  6  4  5”

That means the non-inverting pin is called “3” and its netlist order is 1. The inverting pin is called “2” and its netlist order is 2, etc.  Once that is set up, you can run the simulation.

With the circuit designed and simulated it is time to lay out the PCB. Unfortunately for you, I use EasyPC at work, and as that’s what I will be using to lay the PCB out in. I have given you the tools to create your own flashing light project using an op-amp as the starting point. Next article I will be laying out the PCB, and designing an enclosure for it.

Before I go, I wanted to run through the other two options. The PIC is based around the PIC18LF14K22. This is a low power device, and I happen to have some left over from my electronic dice project (still waiting for PCBs). Not much to say here. I’ve connected up the header pins for the ISCP, and using a 560R resistor I’ve connected both pins of the LED to the PIC (anode to digital pin, cathode to analogue pin – diode “points” to the analogue pin). The 555 circuit used a 560k resistor, 15k resistor, and 10uF capacitor to give a period of around 4 seconds, and duty cycle of 97%. By reversing the diode and connecting to Vdd the LED flashes briefly for 105ms, and stays off for 4 seconds. Again, the 555 is just too power hungry, and the PIC is a bit of a waste of technology. I admit, I could use a PIC10 or PIC12 and get better usage, but I left assembly behind more than a few years ago and I’m not planning on looking back.

As luck would have it the LCM7215’s have just arrived on my desk. And yes, the 5 SOT-23 devices did come packaged in an A4 envelope.

# Stereotypical beginners electronics project

The In-Laws have just moved house. I say move house, but what I mean is sold their old self-build, bought a field in deepest darkest Norfolk and are planning to self-build again. This is great news because it means they’ve got a blank canvas to fill with numerous eco-build projects. The first of which is a criminal deterent. I’m sure the working ip-camera will be used at some point but until then a decoy camera will have to suffice. Decoy cameras work fine so long at they remain believable, and so this decoy camera needs that little extra bit of polish; a flashing LED.

The flashing LED project is the “Hello World” for those whose programming language is solder. Any time you learn a new microcontroller, you will face the flashing LED tutorial. This is designed so that you can set up all the important registers and interface with the outside world. When you talk about flashing LED projects without using a mcu, people will generally offer the 555 timer as a solution. I’m going to look at three options, and hopefully have a working flashing light at the end.

The main design considerations are the need for a long battery life, and a fairly cheap design. I’m going to be using the LPKF Protomat C60 at work to cut the board out, and I’ll be trying to use components I have in my bits box, or can get free samples of. I am aiming for the design to fit on the back of my selected battery solution, and have a suitable mounting fixture integrated into the PCB design.

I know I need to use battery power, and once installed the batteries can’t really be changed. The build should take 2 years, and so I am aiming for more than that. I’m going to overengineer this aspect horendiously as my professional integrety is at stake. Obviously, a car battery would be a rediculous solution, and although space is not at a premium I’d still like to make the package small. My options consist of a single PP3 battery, a single CR2032, or a pair of AA batteries. The CR2032 only has a capacity of around 200mAh, with the PP3 only doubling that. The AA has to be the victor at well over 1000mAh. Thankfully, the AA battery holder is a also almost half the cost of a CR2032 holder. Lastly, is the actual flashing LED. A flashing light project without a flashing light simply isn’t. The minimum current draw I can reasonably expect to look at is 1mA. Any lower and I dont think the LED would be bright enough to see at a distance. The human visual cortex holds onto an image for about 1/15s, or approx 66ms, so I think that 100ms is a reasonable on time for the circuit.

I’ve mentioned the 555 timer as a possible solution so lets examine that a bit closer. Normally, the 555 doesn’t output a duty cycle of less that 50% which is bad for a low power design. The duty cycle issue can be addressed with a 1N4148 in parallel with the resistor between the discharge and trigger pins. Alternatively, you can reverse the LED and connect to Vdd instead of Vss, sinking current through the 555. The ICM7555 is Maxim’s low power derivative of the 555, having a typical supply current of 60uA. Assuming a battery capacity of 1000mAh, the 555 would run for 16666.67 hours (a month less than 2 years) without flashing an LED. The 555 just doesn’t cut it.

The next obvious solution would be a low power microcontroller. This is great as the LED can do more than just flash. It can reverse bias the LED to work as a light detection circuit, and the LED can do a double flash for extra realism. The PIC18LF13K22 has a low power sleep current consumption of only 460nA. This would be 34nA but I need to keep a timer running to wake the thing up. Even a draw of 460nA  gives a run time of 248 years on a 1000mA battery, which is just crazy. Adding the LED gives a battery life of 5 and a half years. With a BOM of only 5 components this ticks every box, but using a microcontroller feels like cheating. I think this will be my backup plan if the next idea falls through.

My final option is an op-amp. The op-amp in question is Linear Technology’s LMC7215. By using positive and negative feedback and a crafty 1N4148, the LMC7215 can be made to oscillate with a definable peroid and duty cycle. I used LTSpice to simulate the actual circuit and can extract the predicted current draw (tutorial to follow), giving a battery life of just over 5 years. Using my parts box, I managed to get a on-time of 92ms, and a period of 4 seconds.

The op-amp circuit does give a higher parts count, but this still ticks every box, and I think it is more elegant than the PIC solution. Next article, I will be showing how to use LTSpice to come up with the appropriate values, and importing SPICE definitions into the circuit. I will also lay out the PCB in CAD, and start the enclosure design. See you then.