Build A Synchronous Clock

The quartz clocks which have dominated time-keeping for the past 20 years or so have one problem: their errors, although slight, are cumulative. After running for several months the errors can be significant. Sometimes you can correct these if you can slightly tweak the crystal frequency but otherwise you are forced to reset the clock at regular intervals. By contrast, mains-powered synchronous clocks are kept accurate by the 50Hz mains distribution system and they are very reliable, except of course, when a blackout occurs. This circuit converts a quartz clock to synchronous mains operation, so that you can have at least one clock in your home which shows the time. First, you need to obtain a quartz clock movement and disassemble it down to the PC board. For instructions on how to do this, see the article on a "Fast Clock For Railway Modellers" in the December 1996 issue of SILICON CHIP. Then isolate the two wires to the clock coil and solder two light duty insulated hookup wires to them (eg, two strands of rainbow cable). Drill a small hole in the clock case and pass the wires through them. Then reassemble the clock case.

Circuit diagram:

A Synchronous Clock Circuit Diagram

To test the movement, touch the wires to the terminals of an AA cell, then reverse the wires and touch the cell terminals again. The clock second hand should advance on each connection. The circuit is driven by a low voltage AC plug pack. Its AC output is fed to two bridge rectifiers: BR1 provides the DC supply while BR2 provides positive-going pulses at 100Hz to IC1a, a 4093 NAND Schmitt trigger. IC1a squares up the 100Hz pulses and feeds them to the clock input of the cascaded 4017 decade counters. The output at pin 12 of IC3 is 1Hz. This is fed to IC4, a 4013 D-type flipflop, which is connected so that its two outputs at pins 12 & 13 each go positive for one second at a time. As these pulses are too long to drive the clock movement directly, the outputs are each fed to 4093 NAND gates IC1b & IC1c where they are gated with the pin 3 signal to IC4. This results in short pulses from pins 3 & 10 of IC1 which drives the clock via limiting resistor R1. The value of R1 should be selected on test, allowing just enough current to reliably drive the clock movement.

Author: A. J. Lowe - Copyright: Silicon Chip

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Petrol Gas Switch For A Pajero

My current vehicle, a Pajero, was modified for dual fuel - ie, petrol and gas. However, its necessary to run the vehicle on petrol at regular intervals to stop the injectors from clogging up. This simple circuit allows the vehicle to be started using petrol and then automatically switches it to gas when the speed exceeds 45km/h and the brake pedal is pressed. Alternatively, the vehicle may be run on petrol simply by switching the existing petrol/gas switch to petrol.

You can also start the vehicle on gas by pressing the brake pedal while starting the vehicle. The circuit is based on an LM324 dual op amp, with both op amps wired as comparators. It works like this: IC1a buffers the signal from the vehicles speed sensor and drives an output filter network (D1, a 560kO resistor and a 10µF capacitor) to produce a DC voltage thats proportional to the vehicles speed.

Circuit diagram:

Petrol Gas Switch For A Pajero Circuit diagram

This voltage is then applied to pin 5 of IC1b and compared with the voltage set by trimpot VR1. When pin 7 of IC1b goes high, transistor Q1 turns on. This also turns on transistor Q2 when the brake pedal is pressed (pressing the brake pedal applies +12V from the brake light circuit to Q2s emitter). And when Q2 turns on, relay 1 turns on and its contacts switch to the gas position. Trimpot VR1 must be adjusted so that IC1bs pin 7 output switches high when the desired trigger speed is reached (ie, 45km/h). In effect, the speed signal is ANDed with the brake light signal to turn on the relay. The vehicle has been running this circuit for several years now and is still running well, with no further injector cleans required.

Author: J. Malnar - Copyright: Silicon Chip Electronics

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Power Flip Flop Using A Triac

Modern electronics is indispensable for every large model railroad system, and it provides a solution to almost every problem. Although ready-made products are exorbitantly expensive, clever electronics hobbyists try to use a minimum number of components to achieve optimum results together with low costs. This approach can be demonstrated using the rather unusual semiconductor power flip-flop described here. A flip-flop is a toggling circuit with two stable switching states (bistable multivibrator). It maintains its output state even in the absence of an input pulse.

Flip-flops can easily be implemented using triacs if no DC voltage is available. Triacs are also so inexpensive that they are often used by model railway builders as semiconductor power switches. The decisive advantage of triacs is that they are bi-directional, which means they can be triggered during both the positive and the negative half-cycle by applying an AC voltage to the gate electrode (G). The polarity of the trigger voltage is thus irrelevant. Triggering with a DC current is also possible. Figure 1 shows the circuit diagram of such a power flop-flop. A permanent magnet is fitted to the model train, and when it travels from left to right, the magnet switches the flip-flop on and off via reed switches S1 and S2.

Circuit diagram:

In order for this to work in both directions of travel, another pair of reed switches (S3 and S4) is connected in parallel with S1 and S2. Briefly closing S1 or S3 triggers the triac. The RC network C1/R2, which acts as a phase shifter, maintains the trigger current. The current through R2, C1 and the gate electrode (G) reaches its maximum value when the voltage across the load passes through zero. This causes the triac to be triggered anew for each half-cycle, even though no pulse is present at the gate. It remains triggered until S2 or S4 is closed, which causes it to return to the blocking state.The load can be incandescent lamps in the station area (platform lighting) or a solenoid-operated device, such as a crossing gate. The LED connected across the output (with a rectifier diode) indicates the state of the flip-flop. 

The circuit shown here is designed for use in a model railway system, but there is no reason why it could not be used for other applications. The reed switches can also be replaced by normal pushbutton switches. For the commonly used TIC206D triac, which has a maximum current rating of 4 A, no heat sink is necessary in this application unless a load current exceeding 1 A must be supplied continuously or for an extended period of time. If the switch-on or switch-off pulse proves to be inadequate, the value of electrolytic capacitor C1 must be increased slightly.

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How to Make a Bicycle Horn with Ringtone

The old fashioned mechanical bicycle horns are slowly getting discarded now and the folks are more interested to replace them with the musical horns imitating well as phone ringtones. One such project is discussed in this article. The circuit is very easy to build as it incorporates just a couple of active parts and a few other passive parts. The circuit can be operated with 3 volts DC through two penlight AAA size.

Electronic hobbyists who also own a bicycle will love this project. The proposed idea will help you to get rid of your old mechanical bicycle horn with a brand new loud electronic horn. Its a homemade project - another aspect that will amuse the young folks. Lets learn the whole procedure here.

Circuit Description and Construction Clues

Referring to the figure we can see how simple it is to construct the proposed circuit as it utilizes very few electronic parts. The transistor T1 is an ordinary general purpose transistor, the well known 8050. An 8050 is more powerful than the usual BC547 types and is able to handle current up to 150 mA comfortably. The transistor also owns the property of having greater hFE levels than other similar types of transistor resulting in better amplification of the music, and yes it is there basically to amplify the music source.

The music source here is the incredible IC UM66 which has an embedded piece of music “written” inside it. It just needs a supply voltage of 3 V (not to exceed) to get going. The pin-outs are also pretty simple to understand. The left one is the negative, center one is the positive and the right leg is the output – simple isn’t that?

Once the relevant supply terminals of the UM66 are assigned to their posts, it starts “singing” right away through its output pin. However, this audio level is very low and needs to be amplified before feeding it to the step-up coil. This is done by T1 as explained above and the amplified signal is sent to the coil.

The coil used here actually acts as a step-up transformer and is primarily used for stepping up the amplified music fluctuations from the transistor T1. The coil just like any other transformer as a primary and a secondary sections, however the sections are not isolated, rather are wound as a single winding with the center tap appropriately pulled out at the relevant calculated step.

The primary and the secondary winding leads are identified by measuring the corresponding resistances using a multi-tester. The leads which show lower resistance is the primary winding, and the one which shows relatively higher value is the secondary winding.

Normally the primary section will indicate a value of around 22 ohms while the secondary shows a value of around 160 ohms. The common lead across the measurements is the center tap and goes to the positive supply.

The piezo plate which is responsible for the actual reproduction of the sound is connected across the secondary winding directly. The terminals of the piezo from the central white area and the outer metal rim, both the areas are solderable, however soldering the connection over the inner circle needs great care, make sure the solder tip is lifted as soon as the solder spot is made, otherwise the white ceramic coating will immediately get burnt reducing some efficiency of the device. Another aspect with the piezo element is its installation or the fixing method.

The fixing is done over a plastic dish or cap having some depth (around 5 mm) and an inner elevated step of about 1.5 mm in height and 1 mm in width, covering the inner bottom edge of the cap (see fig). The inner diameter of the cap is such that the piezo just brushes inside the cap and settles over the elevated step. And it’s exactly how the piezo is placed and stuck inside the cap (see figure).

The sticking can be done by some good quality synthetic rubber based glue (as used for sticking rubber and leathers). The opposite surface of the cap has a central hole of some calculated diameter (say around 7 mm) and it determines the loudness of the generated sound from the piezo element. Varying this diameter of the hole can drastically vary the amplification and sharpness of the music intensity.

Once the entire wiring of the circuit and piezo assembly id completed, the unit can be powered using two penlight cells, which gives the required 3 volts to the circuit. Amazingly even with such low power supply the music intensity can be found to be significantly loud and ear piercing.

However the supply must not be exceeded this value because the IC UM66 cannot tolerate anything above 3 volts. Of course the unit can be used with higher supply voltages, up to 12 volts only if the supply to the IC is checked and regulated to 3 Volts by a resistor and a zener network. With 12 volts supply the amplification becomes very high and in fact becomes very compatible with cars for using as musical reverse horns.

Parts List

All resistors are ¼ watt, CFR, 5 %, unless otherwise stated

R1, R2 = 1 K,

T1 = 8050,

Coil = As shown in the diagram,

COB = UM 66 IC or any other similar type.

Piezo = 27 mm, two terminal type, as shown in the diagram.

PCB = Veroboard or any general purpose PCB.

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