Miniature Realtime Controller
Wichit Sirichote, kswichit@kmitl.ac.th
A Miniature Realtime Controller controls
night light, A/C, household appliances with programmable scheduler.
Introduction
This is my long history the device that controls my home's
night light, air-conditioner,etc. The device is a Miniature Realtime Controller.
The circuit uses only three chips, a 89C2051, DS275(MAX232), and 74LS07
open collector driver. The scheduler for time on/off of 6-channel output
can be made by downloading from PC and saved into onchip RAM. Each output
provides a 20mA sinking suitable for driving a homemade opto-triac or big
solid-state relay for heavy load.
Hardware
A circuit diagram of the Miniature Realtime Controller is
shown in Figure 1. A 89C2051 with a low-cost Xtal 3.579MHz runs timer6.hex.
The 6-channel output is P1.2 to P1.7 driving with sink current. A 74LS07
open collector provides approx. max 20mA @12V suitable for driving a homemade
opto-triac shown in Figure 2. Two signal diodes, 1N914, provid simple backing
up supply for the 89C2051. Since RST uses simple RC circuit, there is no
brownout protection and reset switch thus when the controller operated
with battery for long time, the chip may knock. Simply take the battery
out and put them again.
Figure 1: Circuit Diagram of Miniature Realtime
Controller
Homemade Opto-triac
An opto-triac with zero-crossing can be made easily. I have
used an MOC3041 to drive 5A 240V triac. A night light used in my home mostly
be an incandescent lamp, the circuit below works fine. For such heavy load
as air-conditioner, I have used a solid-state relay,one from CRYDOM D2410,
say.
Figure 2: Example of Connecting Solid-State Relay
to the RTC
Software
Timer6.hex
is the hex file for 2051 chip. The original source program timer6.asm
was written in assembly with c32 cross assembler format.
Example of scheduler air.pgm
was written as a text file. Downloading to the controller may simply
do via communication program, Xtalk or any others program but sending each
character should mathed the receiving speed of the controller. For Xtalk,
the speed may reduced by changing CWAIT command. Figure 3 shows an example
of downloading scheduler using command p(program) followed by command r(read)
to check the stored programm.
Figure 3: Checking stored program via Read command
after air.pgm downloaded
Figure 4: Using Help, Enter time & Date
Last updated, 15 July 2542