Building Your Own Digital Clock

The best way to understand the different components of a digital clock and how they work together is to actually walk through the steps of building your own clock. Here we will build just the "seconds" part of the clock, but you can easily extend things to build a complete clock with hours, minutes and seconds. To understand these steps, you will need to have read How Boolean Logic Works and How Electronic Gates Work. In particular, the electronic gates article introduces you to TTL chips, breadboards and power supplies. If you have already played around with gates as described in that article, then the description here will make a lot more sense.

The first thing we need is a power supply. We built one in the electronic gates article. That time, we used a standard wall transformer that produced DC (direct current) power and then regulated it to 5 volts using a 7805. For our clock, we want to do things slightly differently because we are going to extract our 60-Hz timebase from the power line. That means that we want an AC rather than a DC transformer, and we will use a part called a bridge rectifier to convert the AC to DC. Therefore, we need the following parts for our power supply:

A few notes on the parts used: 

• The difference between the AC transformer we are using here and the DC transformer we used in the article on gates is that the AC transformer preserves the 60-Hz sine wave found in 120-volt household current. If you want to use your volt-ohm meter to measure the voltage of an AC transformer, be sure you use an AC voltage range rather than a DC range.

• We use the bridge rectifier to convert the AC to DC. One of the terminals on the rectifier will be marked with a "+" -- from that you can find the minus and AC inputs. There is no polarity to an AC transformer, so it does not matter which transformer lead you connect to which AC lead of the rectifier.

• The 7805 and capacitors are wired just like they were in the electronic gates article.

• The resistor and the zener diode extract a 60-Hz signal from the transformer's sine wave. A diode is a one-way valve for electrons. A zener diode is also a one-way valve, but it also passes electrons in the other direction if they are above a certain voltage. The zener diode therefore turns a 10-volt sine wave into a clipped wave oscillating between 0 and 5 volts. This is perfect for clocking the TTL counters. The 1-K-ohm resistor makes sure that the current to the zener diode is limited so we do not burn out the diode. The diode will have a band painted on one end -- this band should be the end connected to the resistor.

Circuit Diagram

Here's a circuit diagram for the power supply and time base:

As we saw in the article on electronic gates, the power supply is the most difficult part!

To create the rest of the clock you will need:

• At least four 7490 or 74LS90 chips

• At least two 7447 or 74LS47 binary-to-7-segment converters

• At least 20 resistors for the LEDs in the 7-segment displays (330 ohms would be fine.)

• Some normal LEDs

• At least two common-anode (CA) 7-segment LED displays (Jameco part # 17208 is typical.)

• Breadboards, wire, etc. (See this page for a complete list.)

The number of chips, resistors and LEDs you need depends on how many digits you are interested in implementing. Here we will discuss only seconds, so the "at least" numbers are correct.

Simple Electrical Flashing LED Project

FLASHING LED PROJECT

LED stands for Light Emitting Diode

This project is designed as an introduction to soldering, identifying common components, using the resistor colour code and placing components correctly on stripboard. The LED flashes at about 3Hz (3 flashes per second).

Parts Required

• resistors: 470, 1k, 220k • 555 timer IC (chip)

• capacitor: 1μF 16V radial • battery clip for 9V PP3

• red LED (or orange, yellow or green if you prefer!) 
• stripboard: 6 rows × 21 holes

• 8-pin IC holder (a 'DIL socket') for the 555 IC

Instructions

1. Solder the 8-pin IC holder in the correct place on the stripboard.

2. Break the 4 tracks under the IC holder with a track cutter tool. You can allow extra holes if your piece of stripboard is large enough.

3. Use the resistor colour code to identify the resistors which are marked with coloured bands to show their value.

4. Insert and solder the resistors in the correct position, they can be put in either way round, but you must line them up correctly with the IC holder.

5. Identify the other parts, then solder them in the correct position and the right way round. To help you identify the parts please see our page on soldering.

6. Solder the 2 wire links in place around the IC holder, it is easier to use plastic-coated singlecore wire. (The flexibility of stranded wire is not needed for connections like this and the strands can be difficult to push through the small hole).

7. Finally insert the 555 timer IC and connect a battery!

>>Click on the circuit image for clear view<<

Simple Electrical Circuit Project Low cost Automatic Emergency Light

LOW COST AUTOMATIC EMERGENCY LIGHT

Description:

Here is a white-LED-based emergency light that offers the following advantages:

1. It is highly bright due to the use of white LEDs.

2. The light turns on automatically when mains supply fails, and turns off when mains power resumes.

3. It has its own battery charger. When the battery is fully charged, charging stops automatically.

The circuit comprises two sections: charger power supply and LED driver.The charger power supply section is built around 3-terminal adjustable regulator (IC1) LM317, while the LED driver section is built around transistor BD140(T2). In the charger power supply section, input AC mains is stepped down by transformer to deliver 9V, 500mA to the bridge rectifier, which comprises diodes (IN4007x4). Filter capacitor (25v/1000uf)eliminates ripples. Unregulated DC voltage is fed to input pin 3 of IC1 and provides charging current through diode IN4007(D5) and limiting resistor (16ohm)R16. By adjusting preset 2.2K(VR1), the output voltage can be adjusted to deliver the required charging current. When the battery gets charged to 6.8V, zener diode conducts and charging current from regulator (IC1) finds a path through transistor BC547(T1) to ground and it stops charging of the battery. The LED driver section uses a total of twelve 10mm white LEDs. All the LEDs are connected in parallel with a 100-ohm resistor in series with each. The common-anode junction of all the twelve LEDs is connected to the collector of pnp transistor T2 and the emitter of transistor T2 is directly connected to the positive terminal of 6V battery. The unregulated DC voltage, produced at the cathode junction of Bridge(Diodes), is fed to the base of transistor T2 through a 1k resistor. When mains power is available, the base of transistor T2 remains high and T2 does not conduct. Thus LEDs are off. On the other hand, when mains fails, the base of transistor T2 becomes low and it conducts. This makes all the LEDs (LED1 through LED12) glow. The mains power supply, when available, charges the battery and keeps the LEDs off as transistor T2 remains cut-off. During mains failure, the charging section stops working and the battery supply makes the LEDs glow. Assemble the circuit on a general-purpose PCB and enclose in a cabinet with enough space for battery and switches. Mount the LEDs on the cabinet such that they light up the room. A hole in the cabinet should be drilled to connect 230V AC input for the primary of the transformer. I have tested the circuit with twelve 10mm white LEDs.You can use more LEDs provided the total current consumption does not exceed 1.5A. Driver transistor T2 can deliver up to 1.5A with proper heat-sink arrangement.

Simple Project on Electrical Circuit -Automatic Fan Controller

AUTOMATIC FAN CONTROLLER

Procedure

Th1, the 50K thermistor, is a standard type. Mine was a bar or rectangular looking thingy. Available from Tandy/Radio-Shack. Almost any type will do. I experimented with different models from 22K to 100K and all worked fine after replacing the trimmer pot. The one used in the above circuit diagram was a 50K model. This 50K was measured at exactly 25 °C and with 10% tolerance. The resistance increases as the surrounding temperature decreases. Tolerance for my application (cooling a large powersupply coolrib) is 10%. Another name for this thing is 'NTC'. NTC stands for "Negative Temperature Coefficient" which means when the surrounding temperature decreases the resistance of this thermistor will increase. I replaced my thermistor for a 60K hermetically sealed glass type since the environment for my application may contain corrosive particles which may affect performance on a future date. P1 is a regular Bourns trimmer and adjusts a wide range of temperatures for this circuit.

I used the 10-turn type for a bit finer adjustment but the regular type will work for your application. R1 is a 'security' resistor just in case the trimmer pot P1 is adjusted all the way to '0' ohms. At which time the thermistor would get the full 12 volt and it will get so hot that it puts blisters on your fingers... :-)R3 feeds a bit of hysteresis back into the op-amp to eliminate relay 'chatter' when the temperature of the thermistor reaches its threshold point. Depending on your application and the type you use for Q1 and Re1, start with 330K or so and adjust its value downwards until your satisfied. The value of 150K shown in the diagram worked for me. Decreasing the value of R2 means more hysteresis, just don't use more then necessary. Or temporarily use a trimmer pot and read off the value. 120K worked for me.
 

Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil. D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. For my application the 1N4148 was good enough since the tiny relay I used was only 1 amp. However, you can use a large variety of diodes here, my next choice would be a regular purpose 1N4001 or something and should be used if your relay type can handle more then 1 amp. If you like to make your own pcb, try the one above. The pcb is fitted with holes for the relay but may not fit your particular relay. It was designed for a Aromat HB1-DC12V type. The variety and model of relays is just to great. How to mount it then? Well, I left ample space on the pcb to mount your relay. You can even mount it up-side-down and connect the wires individually. Use Silicon glue, cyanoacrylate ester (crazy glue), or double-sided tape to hold the relay in place. Works well. Note that the pcb and layout is not according to the circuit diagram in regards to the hookup of the fans. The PCB measures approximately 1.5 x 3 inches (4.8 x 7.6mm) If you print the pcb to an inkjet printer it is probably not to scale. Try to fit a 8-pin ic socket on the printed copy to make sure it fits before making the pcb...

To view the circuit diagram in big, click on the circuit diagram image.