Article : Andy Collinson
Email :
Answer: From the schematic, read on to find out how.
If a circuit contains less than 20 components, you don't need a separate parts list, you can obtain them straight from the schematic. If the circuit is larger than this, something may be missed, so it is handy if a parts list is available.
Prerequisites:
Before you start to write down the components, there are certain things you must know about each component. This applies to all schematics, not just the circuits on this site. This additional information is listed below :
Resistors:
Unless stated, any resistor in a circuit will be rated at 1/4 watt. This is a standard wattage and all manufacturers supply 1/4 or 1/3 watt resistors. Higher wattage resistors may be used, but are more expensive than the 1/4 watt type. Although seldom stated a resistor does have a maximum voltage rating of 1000 Volts (not by ohms law) but due to the material it is made from. If the circuit is a high voltage circuit, then special high voltage resistors must be used.
Tolerance Unless stated assume all resistors in a circuit have10% tolerance. Buying 1/3w or 1/4w resistors at 5 or 10% in bulk may save money if you are a serious hobbyist.
Capacitors:
There are many different types of capacitors, electrolytics, polyester, silver mica, tantalum bead etc. Some have different characteristics and offer advantages in particular circuits.
Working Voltage The working voltage of a capacitor MUST exceed the working voltage of a circuit. This is of greatest importance with large value electrolytics, where excess voltage may cause the chemicals in the capacitor to overheat or explode. For any given power supply I would recommend that the voltage of the capacitor be at least 3 times higher than the nominal voltage of the power supply.
Tolerance Capacitors vary widely in tolerance, electrolytics may be +/- 20% of their rated capacitance, whereas ceramic plate capacitors can be made to within 5% of their nominal value.
Capacitors in Circuits:
Unless stated on a schematic, I would recommend the using capacitors as follows:
Ceramic Disc: Decoupling logic circuits, or radio circuits
Ceramic Plate: Timing circuits or other close tolerance circuits
Electrolytic: Use in power supplies and audio circuits, or where large capacitance values are required.
Polyester Film: Decoupling circuits, RF circuits
Tantalum: Low leakage circuits, timing circuits
Inductors and Transformers:
An Inductor in its simplest form is just a coil of wire (air-spaced), however it may be wound on a core that is either iron or ferrite to increase its inductance, or wound onto a former to make a transformer. There are also high frequency transformers known as IFT's these will have a particular resonant frequency and best suited to a radio design. Inductors for a radio circuit will always be specified on the schematic. Transformers for power supplies circuits will always be specified on a schematic with the correct choice of primary and secondary ratings.
Diodes and Transistors:
The recommended first choice of transistor or diode will be stated on the circuit. If the component cannot be obtained locally then a substitute part may be used, usually chosen from a catalog or substitute handbook. If choosing an alternative part make sure the ratings match those of the original circuit.
Switches and Relays:
Switches and relays, being mechanical in nature, can be unreliable components so if you want reliability don't buy the cheaper switches. Switch and relay contacts must have voltage and current ratings greater than the load they will switch, this will be stated on the diagram. If not stated then use a switch or relay with contacts that can handle the voltage and current of the circuit.
How to Create Parts List from the Schematic.
Example 5 Zone Alarm Circuit
My advice is to print off a copy of the schematic then to systematically list all components of one type. Start with resistors, write down their values from the schematic and place a pencil mark against the component on the schematic. Repeat until all components have been ticked. So reading from the schematic:
There are 6 100k resistors, R1, R3, R5, R7, R9, R14.
6 1k resistors, R2, R4, R6, R8, R10, R12.
1 220k resistor R11
1 10k resistor R13. All resistors will be 1/4 watt at 5 or 10& tolerance.
5 100n capacitors, C1 to C5. These may be polyester or disc ceramic, 50V working or higher.
C6 1 100uF capacitor. This should be electrolytic have a working voltage of at least 25Volts or higher.
C7 1 1uF capacitor. Electrolytic as for C6.
3 1N4148 Diodes, D1, D2, D3 which should be readily obtainable.
1 1N4001 diode D4.
5 LED's LED1 - LED5. Colour is not important but you may like to use the same colour for zones 2 to 5 and a different colour for zone 1, the entry, exit delay.
1 2N3904 transistor, Q1.
1 4050B CMOS IC for IC1. Note that CMOS 4050BE may also be used.
1 4072B CMOS, IC2
1 4082B CMOS IC, IC3. Note that unused inputs on IC's 2 and 3 should be connected to earth and that the power pins must be connected. IC pinouts can be found here.
1 Relay with 2 changeover contacts. The coil needs to match the circuit, i.e. 12V coil, the relay contacts must be suitable for the load. As the load is a piezo buzzer, there will be little load current, so a miniature or sub-miniature relay may be used.
1 keyswitch
1 NO PBS (for the panic switch).
1 reed relay ( for the re-entry switch).
5 NC contact switches. These can be bought from alarm shops and fitted to doors or windows etc.
That completes the full list of components for this circuit. It is an easy matter, and can be applied to any circuit. With experience you just look at the circuit and order all the components required.
