7. A Circuits Cookbook
• A cookbook is intended to provide enough examples of useful circuits to fill in black boxes in designs.
• Before using this section, the designer should already have some concept of what they want their circuit to do, and have a block diagram of function. In this section you can find ways to fill the black boxes.
• Although you may be familiar with safety, it is always worth a review. A few careless minutes in a lab can be fatal.
• Some (BUT NOT ALL) safety rules are,
1. Always think about what you are doing, don’t try something without understanding the consequences.
2. Cover or insulate live contact points when not testing, and use insulated probes.
3. Keep objects shielded and properly grounded, and avoid ground loops.
4. Wet surfaces can make you a convenient path for electricity
5. When soldering remember the molten solder is hotter than boiling water, and can splatter/spray/etc.: wear safety glasses.
6. Remember electrolytic capacitors COULD EXPLODE IF CONNECTED BACKWARDS.
8. Double check, and look for stupid mistakes before turning a device on. Common problems are reversed power supply polarity, short circuits, loose connections.
9. Keep things clean while working , and leave the lab better than you found it.
10. If something has malfunctioned deal with it: report it, fix it, or throw it out.
• The cases come in many forms, but for in-house development the DIP (Dual In-Line Pin) package is most popular, and most chips here are numbered with the same pin convention, unless specified.
• Chips are labeled with part numbers, for example the 74F147, will logically be equivalent to the 74LS147, except that they will have different rated speeds. The ‘F’ signifies fast, and ‘LS’ signifies low speed.
• There are extensive volumes of databooks available for chips, these are typically low cost, and available at any vendor of microchips.
• Many manufacturers make common chips, with the same IC numbers. But, there are also many proprietary chips. Be wary when selecting a non-standard IC, small purchases may be frowned upon by the supplier, making them hard to get in quantities of less than 1000.
• CMOS chips will need pull-up resistors on inputs.
• When TTL inputs have nothing attached they tend to “float high” and will indicate that an input is true.
• There are basic families of standard components to be found. Many of these are marked by terse codes and symbols.
• A resistor will have 4 or 5 bands. The bands that are grouped, or closer to one side of the resistor are the nominal value of the resistor. A single band will be set apart, this will be the tolerance.
• Capacitors quite often have values printed on them. When the values are not clear, there may be a number code used.
• Consider the capacitor with a number code below,
• There are a few popular methods of fabrication
7.5.1 Shielding and Grounding
• Shielding is important for all circuits, it prevents electrical noise from creating false digital signals, and from corrupting analog signals.
• Shielding is accomplished through a number of methods:
sheet metal (iron) enclosures keep electromagnetic interference out, or in.
• Cables can be shielded two different ways:
twisted pairs: two wires that are used for a signal (signal and common) are twisted once per inch or more. As a result, any inductive magnetic field induces a current one way for one twist, and the other way for the next twist: hence canceling out the induced current.
shielding sheaths: cable bundles are often covered by a metal foil, or braided wire to provide a general protection for the cable. This shield is to be connected at one end (not two) of the cable to drain off any induced currents.
• LEVEL DETECTOR LIGHT OR TEMPERATURE: To measure temperatures or light levels against one level. If measuring temperature the device should be an RTD. If measuring light the device should be a photoresistor (LDR). The value of resistor R1 should be selected to be close to the normal resistance of the device. The potentiometer can be used to make fine adjustments.
• RANGE CONTROLLER: Upper/lower range controller. This can be done with a simple a simple flip flop. Two level detector circuits are used for the inputs. The Set value should be the upper range, the reset value should be the lower value. The output can be used to drive a relay, or some other driver.
• SINKING SENSOR TO TTL: To convert a sinking sensor to a TTL input. The ratio of resistors R1 and R2 is determined by the ratio between the sensor supply voltage (normally 24V) and the TTL input voltage (normally 5V). The resisto values should probably be between 1K and 10K.
• MOTOR REVERSER USING RELAYS: A circuit that allows a motor to be turned on in either direction (safely). The motor on relay can be a single pole single throw (SPST), whilew the reversal relay mst be a double pole double throw (DPDT) relay. The relays should be selected to carry the peak motor currents.
This circuit can be used for a load that requires a few amps of power, but is being controlled by a low current TTL output. The transistor must be selected so that it can carry the maximum load current. A heat sink should be used if the device will pass a large percentage of the rated current. Note that the voltage loss across the transistor will be approximately 2V. For a higher current load a Darlington coupled transistor can be used.
• SIGNAL VOLTAGE LEVEL REDUCTION: A higher voltage signal can be divided to a lower fraction using a voltage divider. This is only suitable for devices with high impedance inputs and should not be used to reduce battery voltages for motors, or other similar applications. The values of R1 and R2 should probably be about 10K.
• SWITCHING AN AC LOAD WITH A SOLID STATE RELAY: AC loads can be controlled with a low current DC output using a solid state relay.
• CONNECTING A SOURCING SENSOR TO A TTL INPUT: This circuit will reduce the larger voltage output from a sourcing sensor (typically 24V) to the lower TTL level (typically 5V).
• STRAIN GAGE AMPLIFIER: this circuit can be used as a crude strain gage amplifier. The ratio of R1/R2 should be close to the ratio of R3/R4. The trim pot can then be used to make minor adjustments. The values of the remaining resistors can be selected to give a suitable amount of isolation.