A method for setting the duration of pauses in wiper control devices. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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Devices for periodically switching on windshield wipers have long been known. The proposed one differs from them mainly in the way of setting the duration of pauses between windshield cleaning cycles in intermittent mode. Using the method described in the article, the desired duration can be set or changed at any time without any variable resistors or other adjustment elements.

To change the duration of pauses, just interrupt the current pause by pressing the button. The device will remember its duration, which it uses as a reference for all subsequent pauses. The storage element is a capacitor.

This technique can be repeated as many times as necessary. However, it only allows you to shorten pauses. To make them longer, you need to turn off the windshield wiper, turn it on again after a short period of time and press the button to set the desired duration. After a little practice this is not difficult.

The device described below, operating on this principle, is built on discrete transistors and an operational amplifier. But anyone can implement the same algorithm on a more modern element base - digital chips or a microprocessor.

The diagram of the windshield wiper control device is shown in the figure. Electric motor M1 and controlled by a cam connected to its shaft by a worm gear, limit switch SF1 are elements of the windshield wiper drive of the ZAZ-968 car.

Diagram of the wiper control device (click to enlarge)

Switch SA1 sets the operating mode of the windshield wiper: 1 - off, 2 - continuous, 3 - intermittent. If the intermittent mode is turned on, then in the time intervals when the limit switch SF1 is open (this means that the windshield wiper blades are in the initial, one of the extreme, positions) and the thyristor VS1 is closed, the capacitor C8 is charged through the motor M1 and the diode VD2 almost to voltage vehicle network. During the operating stroke of the windshield wipers, the voltage on capacitor C8, thanks to the closed diode VD2, drops slightly. It is used to power source followers on transistors VT3 and VT4 and op-amp DA1.

Diode VD1 eliminates surges in the self-induction voltage of the motor windings M1. Resistor R2 is designed to discharge capacitors C2 and C5 after the wiper is turned off.

When the contacts of relay K7 are closed, capacitor C1.1 is charged to the voltage that drops at that moment across resistor R8. Through the source follower on transistor VT4, the voltage to which this capacitor is charged is supplied to the non-inverting input of op-amp DA1. The current value of the voltage drop across resistor R8 is supplied to the inverting input of the op-amp through the source follower on transistor VT3. The op-amp is used as a voltage comparator. Positive feedback through resistor R15 creates a slight hysteresis in its switching characteristic. Resistor R11 sets the operating mode of the op-amp.

At the first moment after switch SA1 is moved to position 3, capacitors C5 and C7 are in a discharged state. Then the charging of capacitor C5 begins, and the voltage drop across resistor R8 decreases as this charging proceeds. When it becomes less than the voltage on capacitor C7, the state of the comparator at Op DA1 will change. The pulse generated at this moment by the differentiating circuit C3R3 and amplified by transistors VT1 and VT2 will open the thyristor VS1.

The SCR will close the power supply circuit of motor M1. The engine will start running, and the contacts of the switch SF1, having closed, will bypass the thyristor VS1, which will close as a result. However, the engine will continue to operate until the brushes complete their stroke and the SF1 switch opens. During the working stroke, capacitor C5 is almost completely discharged through resistor R9 and diode VD3.

When limit switch SF1 opens again and there is a pause in the wiper operation, capacitor C5 will begin to recharge.

Theoretically, if the SB1 button has not yet been pressed and the voltage on capacitor C7 is zero, the pause in the wiper operation will never end. But in practice, capacitor C7, albeit very slowly, is charged by the leakage current of the gate of the field-effect transistor VT4. Therefore, sooner or later the sign of the difference in voltage values ​​at the inputs of the comparator will change, which will cause it to operate and open the SCR VS1, which starts the stroke of the windshield wiper blades.

However, it is better not to wait for this, but at the appropriate moment in time press and release the SB1 button, thereby setting the required pause duration. When you press the button, the contacts of relay K1.1 are closed, which will equalize the gate potentials of transistors VT3 and VT4. At the next moment after releasing the button and opening contacts K1.1, the gate potential of transistor VT3 will decrease due to the continued charging of capacitor C5. The gate potential of transistor VT4, thanks to capacitor C7, will remain unchanged. This will trigger the comparator and end the pause. In the following cycles, the duration of the pauses will be close to the programmed one, since the voltage on capacitor C7 will practically not change.

To reduce the pause, you must press the SB1 button again at the right moment. The voltage on capacitor C7, which sets the duration of the pause, will change. To increase the pause, you need to turn off the wiper (put SA1 in position 1) and after a short time required to discharge capacitor C7, turn it on again and perform the operation of programming a pause of the required duration. You can speed up the discharge of capacitor C7 by pressing the SB1 button with the SA1 switch in position 1 or 2.

With the specified parameters of resistors and timing capacitors, the device allows you to set the pause duration from 0,5 to 60 s.

It is impossible to replace the K1.1 relay contacts with a simple button due to large interference and leaks through long wires.

Timing capacitors C5 and C7 should be selected with the lowest leakage current. It is advisable to take capacitor C5 with the highest rated voltage possible, which will reduce the leakage current. Capacitor C7 - polyethylene terephthalate K7Z-17. The leakage current is even lower for polystyrene (K71 series), fluoroplastic (K72 series) or polypropylene (K78 series) capacitors.

Field-effect transistors VT3 and VT4 should be selected with the lowest cutoff voltage. Relay K1 is made from a KEM-2 reed switch, on which 1050 turns of PEL 0,12 wire are wound without a frame. The winding is impregnated with epoxy resin. The relay operation current turned out to be 33 mA.

Considering the operating conditions of the device in a car, parts of large mass and large dimensions must be secured to its board not only by soldering the leads, but also mechanically, for example, with glue.

Author: G. Safronov

See other articles Section Automobile. Electronic devices.

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