Building a device from a schematic (Yet another work in progress)

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A problem we see a lot is that people have a schematic (otherwise knows as a circuit diagram) that they want to build. But they find there are numerous stumbling blocks.

This thread aims to help you find your way past those stumbling blocks and on to building your project.

A Quick Note

This thread will assume you have basic knowledge of how to read a schematic. the assumptions are:

• You can identify what parts on the diagram are the components
• You can identify the connections between the wires
• You can identify which way around components go

If you can't do one or more of these, you need to read a thread I've not written yet called "Basic schematic reading skills". If you're not sure, read section 1 which will give you a quick overview. If it seems too hard, you need to read the other thread first.

Here's what I'm covering:

1. What is a schematic?
2. How does a schematic differ from a built circuit?
3. Choice of components
4. Breadboarding
5. Soldering
6. Dead Bugs
7. Matrix Board
8. Veroboard
9. Printed circuit boards
10. Is it practical?

1 - What is a schematic?

This is a schematic:




You will be OK to continue if you know what is described:

• The values of the resistors
• The values and types of the capacitors
• How to distinguish between the two types of diode
• Which end of the diode is the anode (and how you'd determine this on the actual device)
• What the big box in the middle is, and how you'd figure out what the named connections refer to
• What basic type of transistor is used, and which lead is which (and how to figure that out on the actual device)
• What the lines between the devices mean (hint, it's not literal!)
• Where the power supply connections are

And.. What is not shown or described:

• Connections that are not shown (power supply to the 555)
• Voltage ratings for the capacitors
• Power ratings for the resistors
• Power supply voltage
• The specifications/part numbers for all of the diodes

If none of these things are a mystery, feel free to continue...

And to supply most of those extra pieces of information, they are:

Vcc = 6V
All resistors 1/2W, except for R3 which is 2W
C1 10V or higher Tantalum
C2 50V
D1 is a 1N4002
D2,3 are high power red LEDs, W5AM-HZJZ-1-Z or equivalent.

Sometimes more information is shown on a schematic, and here is another example of the same circuit:


Note that the 555 has been shown as an outline of the actual device (so you can see what pins are connected where).

Also note that the (essential) connection to pin 1 is not shown -- and you should be expected to figure that out!

This circuit is also drawn back to front compared to the previous one (which makes no difference) and also shows the battery and a switch.

2 - How does a schematic differ from a built circuit?

As you have seen above, there can be significant variation in the schematic of a circuit. The schematic uses symbols to indicate the connections between components. It is typically drawn to make the function of the circuit easy to understand.

When it comes to building the circuit, reality gets in the way of the nice neat schematic, and we are forced to contend with the realities of the actual shape and size of components, along with how their pins are laid out. We also need to take into account the limitations of our method of construction.

As an example, here is a picture of a set of components similar to what you might get if you ordered the parts to build this.




The components in the middle are all labelled (except the diode, it's a BAT86). The resistors would normally look very similar. It just happens that the resistors I had of a suitable size were all different types, so they display a range of body colours and size variations that would not be typical if you went and bought them from a shop all at once.

Note that I've made some slight changes to the circuit based on the components I had available. See below.

3 - Choice of components

You might wonder why I chose components like this.

Since I'm assuming that this is your first construction project, or maybe your first project using a new form of construction, I will be sticking with through-hole components.

This still leaves a large number of components to choose from, even if we're just picking out a 33k resistor.

One of the things that you get used to is that there are normally a lot of specifications for components that you can ignore, and others where there are some sensible choices.

For example, I could choose a large 33k resistor which can dissipate 10W of heat (10W is 1/2W or more). But this would be large and bulky, and probably quite expensive. Likewise, I could have selected a resistor with 0.1% tolerance. But if not specified, a tolerance of 5% is usually OK. These days you may find that 1% tolerance resistors are actually more common -- that's fine. If you did your math, you might figure that the 33k resistor would be called on to dissipate under 3/10,000ths of a watt. Why would I specify something capable of dissipating 1/2 a watt? The answer is that they are easy to get, and cheap. Getting resistors that are large enough to read the values can be handy for some of us with tired eyes too. There are other things, like composition; do you want carbon film, metal film, carbon composition, or wirewound? The answer is that any are OK for this. Get the cheapest or most easily available -- it is likely to be metal film these days.

In my case, the choice of components was based on what I had on hand. I didn't have (for example any 1 megaohm resistors).

My choice of components dictated a change in the design. Some aspects are cosmetic, others achieve similar functionality a different way, and others are just for the convenience of this demonstration.

Normally, as a beginner you wouldn't be doing this, but it is an example that it is not really very obvious what can be changed without some experience.

Here is the revised circuit diagram:



This looks a little different, and you may notice the component values have changed.

The IC is shown as a Mitsubishi M51848, which just happens to be a 555 manufactured by Mitsubishi for automotive applications. It's the same thing really.

There are 3 LEDs, and R3 is now 100 ohms. This gives 30mA into the LEDs from a 9V supply. It is a real change to the circuit to (a) allow it to operate from 9V, (b) to allow me to use low power LEDs, and (c) to use component values I had on hand.

The mosfet has changed to a 2N7000. This is a lower power mosfet because (a) I happen to have a stash of them, and (b) due to the reduced current, I don't need a higher powered device.

The diode used (a BAT86) is very different to the diode specified (a 1N4002). However this is not significant because I know that the diode is carrying a very low current and a 200mA diode can be used in place of a 1A diode.

You may notice that R5 (33k) has vanished, R2 has changed from 1M to 470k, and C1 has changed from 1uF to 2.2uF. This looks like a major change, but the effect is minimal.

Changes like those above are more typical of those that occur during the design phase than when you decide to build a project. Perhaps it is best to consider he circuit above the one I always intended to build (this is messy).

Just to make things easier, the circuit diagram now also shows the power and ground connections to the 555, and the pin numbers have been added.

4 - Breadboarding

Building circuits on solder-less breadboards is often the first step when playing with a new circuit, or perhaps a part you haven't used before. It's also ideal for making circuits that you don't want to be permanent.

Circuits built on a breadboard are temporary. You'd never build a final circuit this way.

Here is what you require to build this circuit:



Also included are the basic tools required and some other things not shown on the circuit diagram that you'll need to build this on a breadboard.

The multimeter to the left is just the one I pulled out. $10 (or even less) can get you a multimeter good enough for a project like this.

I've redesigned the circuit to run from 9V (see section above on choice of components), so there's a 9V battery and a battery clip. It's important the leads on the end of this are tinned or you won't be able to push the wire into he breadboard.

The top right hand corner shows a breadboard. It's a lot larger than needed for this circuit, but it's false economy to get a tiny one.

Near the bottom right corner are a couple of jumper leads I use for wiring up my breadboard. These are not necessary, but easier than using scraps of wire.

There is a pair of tiny wire cutters near the bottom right corner. These are indispensable for electronics, and while not strictly required for breadboarding, you will need them for all other methods of construction. For breadboarding I will use then to cut the components from the paper tape they're on (they come in strips on up to 5000 -- although you're not likely to be buying that many at a time). You can pull the paper off, but it leaves a sticky residue (which could compromise the connection in the breadboard) as well as sometimes requiring quite a pull on the leads (which could damage the component. (So, just clip them close to the paper tape, OK?)

This is closeup of a breadboard:



The most important thing to know is the way the connections run inside the board.

The outer (top and bottom) pair of rows have conductors that run from left to right the full length of the board.




Beware that some breadboards have a break in these connectors in the middle of the board. This is usually indicated by some marking on the breadboard. The eagle-eyed amongst you may notice there is a break in the blue line on the top of this breadboard. I can assure you that this is a printing imperfection

These connections are normally used for power supply connections.

I like to place my +ve rail at the top and my ground along the bottom. I normally only use one of the strips. You may find examples with more than one power supply, or where (perhaps for personal prefernce) people place power and ground on both sides.

The connections in the middle section go in the other direction:



I have only shown a few here, bet the same pattern repeats along the whole board.

It is most important to realize that there is a break in the middle. This allows us to place a normal IC into the breadboard straddling the center and have access to all of its leads without any of them being shorted out.

Before I do anything, I make sure I know the pinouts for everything. In this case it meant looking up the datasheet for the 2N7000 as I knew the rest...

When constructing the circuit, order doesn't matter a great deal, but I tend to do things in an order and I cross the components (and connections) off the schematic as I go.

First I inserted the 555.



Note that I don't start hard up against one end as I don;t want to be cramped. This is to the left of the middle as I am going to place key components as much as I can (and it's not a lot) like they are in the schematic.

Note the marking on the IC which shows you where pin 1 is. Once you see this mark, you can tell that pin 1 is on the bottom left corner. Going left to right, the pins go 1, 2, 3, 4. the numbering on the other side is in the opposite direction, so on the top side, from RIGHT to LEFT it goes 5, 6, 7, 8. Think of the numbering going around in a circle.

My next task is to connect the power supply wires. I do this first because they're often not shown on the schematic and I don't want to forget them. In some cases it also provides a measure of protection to the ICs themselves.



I have connected pin 1 to ground (or negative). I have used black wires for these, and I also place it on the bottom of the breadboard so it's similar to the schematic.

Positive is pin 8, and that is connected to my positive rail at the top of the breadboard.

Next I connect all the wires from the IC which connect to other pins or to one of the rails. These aren't components, so it's pretty easy to miss them too!



Note the use of red wires again to connect to the supply rail. It's not essential, but it in mnemonic.

I have used a different colour to connect pin 2 to pin 6.

Next I connect all the components which go from a supply rail to a pin of the IC.



And then the components going between pins of the IC.

And finally the other components...



Note how the LEDs are placed in the breadboard so they are in series.

I used a wire (the purple one) to connect the mosfet's gate resistor to the 555. It was just convenient to do it that way.

Finally, after a check that everything is connected, I plug in the battery and...



The LEDs flash.

You'll note that I have only 2 LEDs connected. My 9V battery was quite flat, so three LEDs was a bit of a stretch for the poor thing.

An advantage of using small batteries (and especially flat ones) is that the current is quite limited and if you've made a mistake you're less likely to kill anything.

Once you know it's basically working, switch to a good battery as some circuits (this one included) are sensitive to the batter impedance and work a little strangely with a flat battery.

If you're using a power supply with a variable current limit, start with it set very low.

5 - Soldering

Any of the methods I mention from here on require soldering.

This article will not cover soldering. Be aware that it's a skill that can be picked up fairly quickly, but is not something that you can do well without some practice.

6 - Dead Bugs

The absolutely worst method of building circuits is the technique of just soldering components together without some form of board or substrate to support the components.

Whilst it has its uses, soldering stuff together like this will make it very easy to short things out, make spotting errors much harder, and be almost impossible to plan.

7 - Matrix Board

The simplest construction involves the use of matrix board.

Matrix board is a thin board with regularly spaced holes in it. Often there is no copper on the reverse side of the board, however there are varieties with a disconnected pad behind each hole.

Components are placed into the board and some form of point-to-point wiring is done on the reverse side. Often the component leads themselves are used to make the connections.

For many circuits, this can result in the physical layout matching the most closely the layout of the schematic.

Circuits on matrix board can not be easily duplicated, and wiring errors are easy to make. Nevertheless, for a 1-off circuit, tit is viable (and cheap).

8 - Veroboard

Veroboard is like matrix board, except all the holes in one direction are connected together.

Construction involves planning a layout, cutting traces (often with a sharp drill bit), inserting components appropriately and soldering them in place. Frequently there is a need for jumper wires to connect rows together.

Veroboard required some planning, and often the layout bears only a slight resemblance to the schematic. Nevertheless, whilst the process is sometimes somewhat laborious, it is possible to duplicate the construction with a fairly low risk of error.

9 - Printed circuit boards

Printed circuit board starts life as a (typically) fiberglass board with no holes and a layer of copper on one (or both) sides.

The process of creating a printed circuit board is long and complex. First a board must be designed, then the design must be transferred to the copper, then the board id etched to remove unwanted copper, and finally the holes for components must be drilled.

Optionally you can place text on one or both sides of the board, holes can be plated through, the board can have internal layers, and solder mask can be applied to help protect the copper and make soldering easier. Oh, you can even gold plate exposed copper!

And at this point you can start soldering in the components!

Whilst it seems very complex, this process lends itself to complex designs, and largely automated construction.

The final circuit often looks nothing like the original schematic.

BobK has kindly documented this process here.

10 - Is it practical?

So, you've got a 555 design, and you've assembled it on a breadboard. Maybe it doesn't work. Even worse, maybe it won't work if you build it into a larger circuit.

The issues here are not going to be covered in any depth, but will serve to give you something to google for

1. Power supply decoupling
2. Input noise
3. Leaky capacitors and/or high resistance timing resistors
4. Low value resistors in the discharge path

Power supply decoupling is the term used to describe how you keep noise or transients on the power supply away from parts of the circuit. Most often it is used to protect the power supply to sensitive electronics. In the case of a 555, it is to keep the noise away from other parts of the circuit. The way it is done is to place a capacitor (perhaps 2.2uF to 10uF) across the power supply near the 555. The 555 can draw high currents briefly while switching and this can affect other parts of a larger circuit. (The capacitor on the Control pin of the 555 acts to decouple the reference voltage from the supply voltage -- whilst not decoupling the power supply, it is also a decoupling capacitor. 0.01uF is often used here)


Input Noise is noise coming from an outside source to an input. With the 555 this affects things such as monostables which have an input (but can also affect other types of circuit if the noise is strong enough!). Some form of input conditioning may be required for monostables (I won't cover that here). For other noise issues, power supply decoupling may be required (see above). A capacitor (0.01uF) on the control pin is often required.


Leaky capacitors and/or high resistance timing resistors - Please see https://www.electronicspoint.com/long-duration-timers-notes-beginners-t244516.html which discusses this in some detail. Normally it becomes an issue when you want long periods and/or very low frequencies.


Low value resistors in the discharge path - The resistor between the capacitor and the discharge pin (in the discharge path if you're using steering diodes) acts to set the timing, but also to limit the current through the 555's discharge transistor. Using a value that is too low can lead to the demise of this transistor (and hence your 555). Refer to the datasheet for guidance here. Note that the other timing resistor is also effectively in the discharge path, and can contribute current too. Neither resistor should be too low in value.



Notes for readers who get this far

I'm going to try to drag up a set of components and build this flashing LED circuit using each of the methods described above (even dead bug maybe).

If anyone wants to assist, the only step I can foresee having great difficulty with is getting a professional looking board manufactured (because I don't generally do that. If anyone is in the position of getting some prototype boards made and can design a PCB for this and place it in an unused corner, I'd very much appreciate it. (We have an offer for someone to do this -- Thanks BobK)

For simplicity, I'm planning to increase the size of R3 and use normal red LEDs

If anyone feels like building up the circuit in any other way and can provide me with photos, I'd also appreciate that. I'm not likely to be able to spend too much time doing this.

It might be worth posting a note that you're planning to help so that we don't get too many people all making a matrix board LED flasher (for example).

If anyone wants to suggest any other methods of construction (perhaps a-la those spring terminal 100-in-1 type kits) or maybe tag strip (for ICs?!?!) then suggest away -- but be prepared to do the actual construction![SIZE=2] One obvious method is the one using stuck on pieces o[SIZE=2]f [SIZE=2]PC[SIZE=2]B, W[SIZE=2]h[/SIZE]ilst this is mostly a form of SMD construction, if anyone wants to do this (or even a full SMD PCB sol[SIZE=2]ution![/SIZE][/SIZE][/SIZE][/SIZE][/SIZE]
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[SIZE=2][SIZE=2][SIZE=2][SIZE=2][SIZE=2][SIZE=2]The text is also clearly insufficient, so if you can point to good web sites, or[SIZE=2] write some good text, I'll gladly incorporate [SIZE=2]it.[/SIZE][/SIZE][/SIZE][/SIZE][/SIZE][/SIZE][/SIZE][/SIZE]
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That's great. I was going to add links myself, but you beat me to it.

I'll add them in the appropriate sections when I have a little more time.

 

Yeah, thanks. That's a very good point. Power supply decoupling can be an issue with 555s.