Introduction: From Schematic to Protoboard - Building a Simple LM386 Guitar Amp on a DIP Protoboard
Well hello there gentle reader. Today I am going to explain the steps that I go through to take a circuit idea from schematic to the prototype board. Much in the same way that musical notes go from a flat cold piece of sheet music to the sweet sweet dulcet sounds of the Sousaphone. There is an method that is followed from beginning to end. A complex process is broken down into simpler steps, each step building onto the one preceding until the whole is achieved.
For this project I will show you how I build a small battery powered LM386 guitar amplifier. This amplifier was featured in another Instructable: https://www.instructables.com/id/DIY-Jelly-Jar-Gui...
There is a companion video to this Instructable that'll take you though the build step by step:
Now lets get started with the project. First lets review the parts list......
Step 1: Parts List
Here is a list of the parts you will need:
1 9V alkaline battery
1 9v battery snap connector cable
1 mono 1/4 inch jack with nc (shunted) switch ( I like the Switchcraft 12A)
1 stereo 1/4 inch jack ( Switchcraft 12b)
1 LM386 amplifier. Any version is fine, I like the LM386N-4, but I'm kinda funny that way.
1 66mm Mylar Cone Speaker (MCM #SR00187)
1 Red LED
1 470 Ohm
1 1K Ohm
1 10 Ohm
Protoboard. I use a Radio Shack DIP protoboard (276-159B).
Soldering iron and solder
Step 2: The Radio Shack DIP Protoboard
For this project we will build the circuit on a Radio Shack DIP protoboard. This protoboard has copper tracks on one side, corresponding to legs from two rows (or rails) of contact holes. A DIP IC Chip is meant to straddle the rows of contacts and the legs allow several connections to be made to each pin of the IC Chip.
The components of the circuit board are assembled on the plain (non-copper) side and the components are soldered to the board on the copper side.
It is hard to visualize the tracks while working on the plain side of the board, so for the beginner, it is often helpful to use a Sharpie to outline the tracks.
It is also helpful to use an illustration of the protoboard to plan where each component in the schematic into be placed. As shown above, the illustration of the protoboard is seen from the plain side with the bottom copper side represented in relief.
When you are planning where to place components it is a whole lot easier to sketch their position on printed copies of the protoboard illustration and make your mistakes there instead of ruining a real protoboard when things don't fit right. Let's have a look at the process.....
Step 3: Translating the Schematic to the Protoboard
So once you have designed your circuit and made a schematic, its time to start to work on the layout of the protoboard. So you take pencil to paper and start laying out the components on the protoboard illustration.
This takes some finessing. It'll take some time. It'll take a lot of paper, a lot of cursing, some Jack Daniels and taking the lord's name in vain. But you'll finally get it.
Above you see the schematic and the finished outline on the protoboard illustration.
When I am planning the location of components on the protoboard, I do my planning in stages. I'll start with the placement of the chip. I work left to right so I lay things out like this:
INPUT ---- IC CHIP ---- OUTPUT
The input of this circuit is pretty simple, so the chip will be placed towards the left hand side of the protoboard. This leaves room for the output components.
The next stage is to lay out all the power to the circuit.
The next stage is to lay out the input and output components.
The next stage is to add the gain components and LED power indicator
The next stage is to add the peripheral components like input and output jacks.
Each stage builds upon the last. From the simplest to the more complex.
Doing the build in stages allows you to test and troubleshoot as you go so that if there is an error, it is more likely that it is caught early and can be isolated from the previous stages in the build.
Once the schematic has been translated onto the protoboard illustration it is time to build the circuit for real.
I break the protoboard illustration down onto several sheets that each contain one stage or part of the build. Therefore I can print out each section of the build and check off the components that I place on the real protoboard as I go along so that I can make sure that nothing has been missed. This is crucial especially on complicated builds.
So next we will go over each part of the build. I will show you the protoboard illustration for each part and we will discuss how to troubleshoot before the next step.
For this particular circuit we will be doing the build in 4 parts.
Part 1: Placing the IC Chip and Power Connections
Part 2: Adding the Input and Output Stages
Part 3: Adding the Gain Stage and LED Indicator
Part 4: Adding the Input and Output Jacks
So lets get going .......
Step 4: Part 1: Placing the IC Chip and Power Connections
The protoboard illustration for this stage is shown above.
After the IC Chip is placed it is time to start to add the power connections.
In this circuit pins 3 and 4 are connected to ground. We will concentrate all of our ground connections in that area and we will build a set of interconnecting ground rails. We jumper pin 3 to pin 4 and we jumper from pin 4 to the leg immediately to the right. That way we have plenty of places to connect circuit components to ground.
We then place the positive and negative battery leads to the bottom right of the protoboard. Power to the circuit is controlled by opening and closing the ground connection to the negative terminal of the battery. Therefore we place a lead (green wire) from the leg connected to the negative battery terminal to the switch in the input jack. We then run a wire that will be connected to the ground side of the switch on the input jack to the leg on pin 3. Therefore when a guitar cable is plugged into the input jack it will connect the green and black wires and therefore connect the circuit ground to the negative battery terminal and current can flow. When there is no guitar cable plugged into the input jack, the connection between the green and black wire will be broken and no current can flow in the circuit.
Now to the positive or V+ connection to the circuit. This is shown in red in the illustration above. Power to the IC chip goes to pin 6. This connection is made through two jumpers, one bridging across the rails and one from that leg to pin 6's leg.
To complete the power connections there is a 220uF capacitor placed across the battery terminals to provide stable DC power to the circuit. This capacitor is polarized. The positive leg of the capacitor goes to the positive battery lead and the negative leg to the negative lead.
We will forgo troubleshooting this section until we have the input and output sections connected. So check your work. Make sure that your connections match those on the illustration.
Step 5: Part 2: Adding the Input and Output
The input signal for this circuit comes from the +ve lead of the input jack and goes through a 100nF capacitor and then to pin 2. So we will connect a 100nF cap to pin 2, jump it over pin1 to the next leg over. A red input wire is soldered to the same leg. That's it for the input.
The output signal originates from pin 5. The output goes through a Zobel network to improve output stability. The network is constructed as follows: a 10 Ohm resistor comes off of pin 5 and connects to the next leg to the right. A 47nF capacitor is connected to that leg and jumps to the ground rail. Also from pin 5, a 220uF capacitor is crossed over two legs and soldered. A red wire is connected to the other leg of the capacitor and runs to the +ve lead of the speaker. The capacitor is oriented so that the positive lead is connected to pin 5. The negative lead of the capacitor is connected to the red speaker wire. The negative (black) speaker lead is run from the ground rail.
Now we can test the circuit. The output leads are connected to the speaker. On the input side we will connect the black and green wire together, mimicking the input jack switch. We then will connect a 9V battery to the battery strap.
There should be no sparks or smoke. If there are no sparks or smoke, pat yourself on the back. You did good.
Now comes the real test. Grab hold of the red input wire. Hear that humming? Thats the sweet sound of success! Pat yourself of the back again.
If you don't hear any humming, check your work. Make sure all the power connections are correct. Make sure that all your solder joints are good. Have a look at the illustrations. Watch the video. This will work.
Once you are satisfied that you have the circuit working we can move on to the next step.
Step 6: Part 3: Adding the Gain Resistor and LED Indicator
Ok. We're almost there. Lets add some gain to this puppy.
The gain of the circuit is controlled by pins 1 and 8. The amp has its lowest gain when there is no connection between pin 1 and 8. The maximum gain occurs if we put a 10uF capacitor between pins 1 and 8. Foregoing the capacitor and placing a resistor between pins 1 and 8 give intermediate gain.
The nice thing about the LM386 audio amplifier is how it breaks up when it is overdriven. With low gain, the sound coming out of the amp if clean. With full gain, you'll always be overdriving the amp and never have a clean signal. If thats what you like, by all means build it that way. I like a choice.
In this simple amplifier circuit, the size of the input signal to the amplifier is controlled by the volume knob on the guitar. I like my amp clean with low volume signals and crunchy with high volume signals. Therefore after experimenting with different resistor values I decided that I liked the sound with a 1K resistor jumped across pins 1 and 8. If you like this too, great. If you want to experiment, by all means try different values until you're happy.
The last thing we're going to add to the circuit is a red LED as a power indicator lamp. If you want green, or blue, or clear, or yellow, that's your choice. I like red. That's what I used.
So the LED goes across the V+ rail and the ground rail. But you can't just put an LED across 9V. If you do, the LED will go voot, a tiny puff of smoke will come out of the LED, and a unicorn in a distant land will shed a tiny tear.
You need to use a current limiting resistor. I use a 470 Ohm resistor. Attach the 470 Ohm resistor to the V+ rail. jump it to the next empty leg. Then attach the +ve lead of the LED (the longer lead) to the 470 Ohm resistor. Attach the negative (short) leg to the ground rail.
Now back to troubleshooting. Attach the speaker leads to your speaker. Attach the green and black inout jack leads together. Snap a 9V battery to the battery strap. The LED should light up. If it doesn't you've put the LED in backwards. Fix it. Fix it now.
If the LED is shining like a raccoon's eyes in a rain gutter, you're a genius. Now grab onto the red input wire. Sound louder this time? Well it should. Cause you have more gain. Be proud. Be real proud. Show your mama what you've done. Take a pic for Facebook.
OK so lets finish up this puppy by attaching the input jack.
Step 7: Part 4: Adding the Input and Output Jacks
Lets start with the input jack. We will use a 1/4" stereo jack for inpu . We do this so that we can use the jack itself as a switch to connect or interrupt the battery ground depending on whether or not there is a guitar cable plugged into it. The stereo jack has three contacts. The tip, the ring, and the sleeve. A mono guitar plug has two contacts, the tip and the sleeve. When you plug a mono guitar plug into a stereo jack, the guitar plug sleeve makes contact with both the sleeve and ring on the jack, effectively shorting those leads together. Therefore if we place the green wire on the ring and the black wire on the sleeve, we have effectively made a power switch. When there is no plug, the connection between the green and black wires is open and there is no connection between the ground rails and the battery -ve terminal. When a plug is inserted, the green and black wires are shorted together and now the circuit is complete and current can flow. Brilliant ain't it. So go ahead and connect the input wires are follows: Red - tip, Green - ring, Black - sleeve.
Now we troubleshoot. Hook the speaker up and snap a 9V battery into he battery strap. The red LED should be off. Now plug a guitar cable into the input jack. The red LED should come on! Success!!
Now strum the guitar a bit. Sounds good doesn't it. Turn the volume knob down. The tone out of the amp should be clean. Turn it up, the amp will start to break up. Tadaaa....... You did it!!!
Now this amp is a practice amp. It won't blow out the walls. But its plenty loud to practice your devil music in your basement. Any louder and Meemaw will call the cops on ya. Now you don't want that.
So what do you do with the amp? You can keep it just as it is, or you can put in something. I put mine in a jelly jar. That's just how I roll. Look at the Instructable link I referred to at the beginning of this opus. If you don't like the prospect of shards of glass flying around when you get all Nugent on your guitar, you can put the amp in a cigar box. Heck, you can even put in "Mr Wubby", your teddy bear from when you were 4..... just saying.
So this schematic has an output jack included in it. Why would you want to use an output jack? Well you don't have to. But if you want to use a bigger speaker or group of speakers, say a Marshall 4X12 cab you can plug a cable from the output of this amp into it. Will you play a stadium? Na ya big dummy its only 1/2 watt. But you'll get a pretty big sound out of it on a big speaker system. Worried about impedance matching? well don't. This thing will drive anything from 4 Ohms to 16 Ohms.
So this is how you hook up the output jack. We will be using a 1/4" switched (normally closed) jack for the task. This jack has three connections. The tip, the sleeve, and a switch. We'll attach the output leads as follows. The ground from the speaker output will go to the sleeve. We'll run another black wire from the sleeve to the negative lead on the built in speaker. The red wire that we've connected to the +ve terminal of the built in speaker will now go to the tip. We'll get a nice fresh green wire and connect it from the switch to the +ve input of the built in speaker.
This is how it works. When there is nothing plugged into the output jack the green wire will be connected to the red output wire and sweet sweet music will come out of the small built in speaker. However when we plug in an output plug, the connection between the green wire and red wire will be broken and no sound will come out of the built in speaker. Now all the sound will come out of whatever speaker is connected to the output plug.
Step 8: Conclusions
So gentle reader we have come to the end of our journey.
You should now be able to decipher this simple schematic and translate it step by step to a protoboard. If you can do this simple circuit you certainly can transfer this knowledge into prototyping more complex circuits. Just break the build down into a series of simple steps.
We did this on a DIP style protoboard but you could do it on any style of protoboard if you just follow the steps outlined above.
It's really that easy.
Now go out and build something!!!!!
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Please be positive and constructive.