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by Bill Mills
On a schematic, a resistor is a squiggly line. It is not current specific (power can run through it in either direction). There are also variable reistors. These are resistors that can be adjusted to provide more or less resistance, just like a water faucet that can limit the flow of water out of a pipe. A common application of variable resistors is the volume knob on an older stereo, or a dimmer switch for a light bulb. A variable resistor is represented in a schematic as a resistor symbol with an arrow at an angle through the center.
Some variable resistors have three terminals. The center lead is common, and turning or sliding (depending on the type) the control knob in one direction will increase the resistance to one lead while decreasing it to the other. These three terminal variabile resistors are represented as a resistor with the common terminal as an arrow pointing to the center.
Just like power sources, resistors can be wired in series (end to end) or in parallel (side by side). When wired in parallel more current flows across the leg with the less resistance. Current flow must be calculated for each of the parallel resistors and then added together. When the resistors are wired in series, the resistance is simply added together.
Rtotal = R1 + R2 + R3 ….
So, if a circuit calls for 10,500 Ohms of resistance, that can be achieved by putting a 500 Ohm resistor in series with a 10K resistor.
Things get more complicated when we start mixing and matching series and parallel patterns together, and take up the better part of a few lectures in an introductory electronics course. The good news is that for microprocessor based control circuits on a paintgun, we don’t need to know much more than this to b e very productive.
A capacity for charges
Another key electronic component is the capacitor. In the simplest terms a capacitor is nothing more than a pair of plates with a non-conductor between them. The plates can be charged, and later discharged. Building electrical potential in a capacitor is referred to as biasing it. Capacitors play a very important role electronic frequency generating and filtering circuitry and come in a wide variety of shapes and sizes, depending on their particular materials. Fortunately basic input and output circuits to a microprocessor won’t rely on capacitance, so we can pretty much skip right on past capacitors. The main place they would come into play is filtering “switch bounce” out of trigger signals (which can also be done in software) and dampening radio interference generated by motors. Capacitance is measured in Farads, and the most practical unit is the MicroFarad, or uF, often simply written as u. Depending on the type, capacitors may not be current specific, or may have one side meant for positive bias – this will be indicated in any schematic which requires a capacitor, and the case will in some way indicate the positive or negative lead.
A magnetic personality
Magnets are especially important to us, as electromagnetism is one of the most practical ways to convert electrical energy into mechanical movement. When electrical current flows through a coil of wire, it generates a magnetic field. This field can push or pull against a magnet causing it to move.
Alternating fields in coils of wire are what make electric motors spin. Solenoids are simpler electromagnetic devices. They are just a coil of wire, and when current passes through them, they pull a magnet toward their center. A common place they are found is in the electronic door locks of cars – and in paintball guns. Some paintguns, like the Airgun Designs E-Mag use a large heavy duty solenoid to operate the mechanical portion of the paintgun (the sear on the E-Mag would moved by the trigger on a standard Automag). While this is a great approach for products like the Centerflag Hyperframe, where the solenoid is added to an existing mechanical system, the solenoid is having to do a lot of work. The more “pull” the solenoid needs to operate, the more electrical resistance it creates, and the more wattage is needed to get the job done. Many other paintguns take a different approach. The Angel, Shocker, Matrix, Bushmaster and others use small solenoid valves. In these pre-fabricated valves, a small solenoid is used to operate a small spool valve (very much like the 4 way valve in an Autococker) to re-direct control gasses that drive a pneumatic cylinder. In this way the electrical circuit is only doing a small amount of mechanical work, and letting the paintgun’s gas supply do the real work. The circuitry and power supply needed to drive a small solenoid valve does not need to be as rubust as that for a larger mechanical drive solenoid.
The schematic symbol for a solenoid is a coil with a core in the center.
An important thing to know about solenoids is that when the current driving them is released the magnetic field it created will collapse and the collapsing field will generate an electrical current in the opposite direction. Especially with sensitive microprocessors in a circuit, it is important to design circuits that prevent this backwash of current from destroying part of the circuitry. And, for the record, the word is solenoid - S O L E N O I D, not "selenoid." I don't know why, but I get a couple of e-mails a week from people asking about "selenoid" valves.
Turn me on!
Switches are quite important. Switches and pushbuttons come in an unbelievable array of sizes, shapes and styles. The important things to know about them are the style, and contacts. Basically a switch either makes or breaks an electrical connection by touching conductors together. When the contacts are touching, current can flow from one side of the switch to the other and the switch is said to be closed. When the contacts are open and no current can flow, the switch is open. Momentary switches return to their off-state when they are released. The typical button that controls a car horn is a normally open momentary switch. When it is pressed it closes a circuit delivering power to the car horn, and when it is released the circuit breaks again. This is the type of push button that is needed to receive an electronic signal from a trigger, or for use as a mode-select button. A single pole, single throw switch is what is used for a light switch in a home. It rests in either an open or closed position, delivering current or not, depending on whether it is on, or off. SPST switches are used for power switches in paintguns and other small electronics. Switches are represented in schematics by diagrams that show the layouts of their contacts.
One way only!
Diodes are electronic one way gates. They let current pass through them in one direction only. In the schematic diagram, there are two circuits with light bulbs. In the circuit on the left, the diode is aligned in the circuit properly, and current flows through it to light the bulb. In the other circuit, the diode is in opposition to the circuit, and it blocks the current, preventing the bulb from lighting.
Putting a diode in series with a solenoid is one way to protect circuitry from the backwash of current when the solenoid’s field collapses. The schematic symbol for a diode is a triangle pointing toward the negative side of the circuit, with a small cross-bar.
LEDs are a specific variety of diodes which emit light. Because of their very low current drain relative to their brightness, they are superior to light bulbs in display and compact electronic applications. Since LEDs are diodes you must remember that current can only pass through them in one direction. The schematic symbol for an LED is the same as for a normal diode, but has small wavy lines coming out of it to indicate light.
Transistors come in such a wide variety of materials, shapes and functions that there are literally whole books about them. Transistors are three lead devices which function somewhat like a valve. In a most general description, they control a large amount of current through two of the leads with the presence or absence of current to the third lead. Often they are used to amplify signals. A weak signal can be used to control a transistor that has a large amount of current flowing through it, and the large current will be an amplified version of the weak signal. From a control aspect, they can used to turn on or off the flow of current to high drain devices. This is what really matters to us when designing circuits for paintguns. A low amperage control current out of a microprocessor can control a transistor which will then drive a solenoid or solenoid valve.
Other special semi-conductors handle tasks like voltage regulation. Just like a gas regulator controls pressure on a paintgun, a voltage regulator can limit the voltage supplied by a battery or battery array.
In no way should
this article be seen as anything close to a complete course in electronics.
It will hopefully, at least give you a starting knowledge that is enough
to work with the simple circuit designs needed to integrate a microprocessor
into a circuit for paintgun control. To really get involved and design
your own circuits, self paced instructional books like Basic
Electronics Theory by Delton T. Horn are available from bookstores,
or local libraries.
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