Understand the 74HC595  (David Caffey)

                              

Step 1 : Theory

The Simple 595 system is designed around a 74HC595N 8-bit shift register IC chip. The 595 electronics are connected to the computers parallel (LPT) printer port with four wires:

*Clock signal that is sent to all the chips (SH_CP)  *Serial Data signal that is sent to the the first chip (DS) *Storage Register Clock Input Data that is sent to all the chips (ST-CP) *Ground

The Clock (SH_CP) signal keeps time like a metronome used by musicians to keep time.

The Serial Data (DS) signal tells the IC chip to grab the next 8 bits.

The Storage Register Clock Input (ST-CP) signal contains the data to instruct the channels to turn on or off.

In the example in Step 2, there are two 74HC595N IC chips connected to the parallel port of the PC (16 channels).

Summary: All channels are off, then on the first scan, channels 0, 1, 12, 13, 14, and 15 are turned on. On the second scan, Channel 0 is turned off.

Details: When DS pulses from the PC, you see that Output 0 and Output 1 on IC chip#1, goes high. Then IC chip #1 has determined that 8 pulses have past and it is time to pulse it's Q7' output which is connected to the next IC #2 chip's DS input. When the DS input of IC #2 gets a pulse from the Q7' output of IC #1, outputs 4, 5, 6, and 7 go high on chip #2. When the PC sends a DS pulse to IC #1 again, IC #1 reads the first eight bits from ST_CP and Output 0 on IC #1 turns off but output 1 stays on because it got a pulse and so on...........

Step 2 : 16 Channel Example

Step 3 : Major componets

The major electronics consist of: * 74HC595 shift register chips * Triac drivers *Triacs

Each 74HC595 chip has 8 outputs that output 5 volts DC if Vcc is 5 volts DC.

The schematic from the computer's printer port to the chips looks like the picture in Step 4:

Step 4 : Wiring Schematic

The Triac drivers use the 5 VDC output signal from the 74HC595 chip outputs (Q1, Q2, Q3, Q4, Q5, Q6, and Q7) to switch the 110VAC gate signal to the Triacs.

The Triacs switch the 110 VAC signal that turns the power ON and OFF

to the string of lights.

Step 5 : Port Voltage Problems

If you are connecting the IC chips directly to the PC, a desktop PC MUST BE USED to drive the electronics in a large system because a laptop computer does not have the voltage required to run a large quantity of 74HC595N chips. A laptop can be used in smaller systems (16 channels) but the voltage to the 74HC595N IC chips will need to be lowered since a laptops output voltage is lower than that of a desktop. Add on PCI cards tend to have a lower voltage. Also don't expect to drive as many IC chips with a laptop as compared to your desktop.

In some cases, the parallel port of a desktop will not provide sufficiently high pulses and the Vcc (5 volts DC) to the 74HC595N chips needs to be lowered so the 74HC595 can "see" the pulses from the parallel printer port. The symptom results in some channels working and others not and channels triggering out of order.

Here is the catch, if you lower the voltage to the IC chips then the voltage from the outputs of the IC chips also gets lowered and the current from the outputs gets lowered. It may be necessary to lower the value of the limiting resistor from the 74HC595N chip output to the triac driver in order to properly trigger the triac drivers.