Production of Silk Screen Exposure Timer

Production of Silk Screen Exposure Timer

Generally, the time required for screen exposure is 8 to 15 minutes. The circuit in the attached figure can set the exposure time arbitrarily within 1 to 15 minutes, with an interval of 1 minute. The integrated circuit IC2CD4516 in the figure is a four-bit binary presettable add/subtract reversible counter. It has four functions: clear, preset, add count and subtract count. It has three control terminals, namely the add/subtract count conversion terminal U/D, the preset terminal PE and the reset terminal R; there are four set terminals DP1~DP4; there are four output terminals Q1~Q4; there are also carry input terminals and carry output terminals. CP is the add or subtract count pulse input terminal. CD4516 adopts a dual in-line plastic package. The corresponding relationship between each functional terminal and the pin number is shown in the truth table (attached table).

In the attached circuit, the square wave oscillator composed of IC1 (NE555) and other components can make the oscillation period 1 minute by adjusting the potentiometer RP. The oscillation pulse is sent from pin ③ to the counting pulse input CP@ pin of IC2. The CI carry input terminal, U/D plus/minus count conversion terminal and R clear terminal of IC2 are all grounded. It can be seen from the truth table that IC2 in this application is in the down counting state. The carry output terminal of IC2 is high level during the down counting process unless the four output terminals Q1~Q4 are reduced to zero. Before each exposure starts, the exposure time is set by the dial switches S1~S4, that is, the preset number for IC2. These four switches follow the binary coding rules and can preset decimal numbers 1, 2, 4 and 8 to CD4516 respectively. For example, if you want to preset the number 5 to IC2, you can close switches S1 and S3. After the preset switches are set, press the start switch QA, the exposure timing starts, and the exposure light lights up. The circuit is powered by a power supply composed of transformer T and three-terminal regulator LM7812. Under the action of capacitor C3 and resistor R2, a short-term high level appears at the PE end of IC2. From the truth table, it can be seen that IC2 is set at this time, and the decimal number 5 determined by switches S1 and S3 is locked at the output end, and the LEDs LED1 and LED3 are lit. Indicates that the exposure timing time is 5 minutes. Since the output terminals Q1~Q4 of IC2 are not equal to zero, the carry output terminal CO is high, the transistor V is saturated and turned on, and the relay J is activated and closed. The relay contacts connected in parallel at both ends of the start button QA ensure the continuous power supply of the power supply. At the same time, the oscillator IC1 starts working and sends a count-down pulse to the CP end of IC2 every minute. The count-down process, that is, the exposure timing process, is indicated by the LEDs LED1~LED4. When all output terminals Q1~Q4 are counted down to zero, the CO terminal of IC2 becomes low level, the relay is released, the power is cut off, the exposure light goes out, and the exposure process ends. The circuit returns to the initial state and waits for another timing setting and exposure process. In fact, the oscillation period of IC1 does not have to be strictly adjusted to 1 minute. It can be adjusted to 50 seconds to 70 seconds, or other appropriate durations according to the exposure timing requirements.

The materials involved in this production are all common components, and there are no special requirements. The transformer T is selected as 3-5W, and the secondary voltage is about 15V; the relay is selected as JQX~13F, and the coil voltage is 12V. The two sets of contacts of this relay are used in parallel, and the load current can reach 10A; the setting switches S1~S4 can be selected as dip switches, or other convenient button switches can be used.