Dear All,
A few points on this project of mine:
1.The triac is expected to slow down ordinary ceiling fans.
2.Only a few steps in dimming will do.Not very slow.
3.This is a low cost application > AT89C2051 but unfortunately the built-in comparator I cannot use for zero crossing since AT89S2051 is not yet in demand in Kolkata,India and I am forced to develop on AT89S52.
4.Here is the picture of components of the original 4.3cm x 3.5cm PCB.
http://www.8052.com/users/AsokRudra/TrRC5_629.gif
It was probably a PIC12C508 with MOC2051,probably because markings are filed away.I tried with PIC12F629 and could do only upto polling switches.Again 12F508 wo'nt be available till the foolish dealers finish their 12C508 stock (Microchip's clever marketing!).So I thought of continuing with AT89C2051 and watch for the pcb size.

4.PICs have interrupts on pin change,51 does not.Hence I had to bring in a 4N35 cheaper/MCT2E costlier.
5.I thought I will be able to hack "Remote light control with Dimmer" of Elektor and "Holiday Dimmer".But alas!
6.I canoot yet C..... I can hardly understand and hack successfully,for example added alarm in Sirichotes clock in SDCC,but not yet written any really.
7.Snubbers were there in the original.Thanks for reminding me.I will modify my circuit.
8.My RC5 is of Juergen Putzger which was reliable till this project.

Now Dear Russel, Can you please let me know the expected phase shift in units of time using the secondary of the transformer? I may have to use it, although I would prefer a transformerless design using a 1µF/400V for our 230 V AC.
(b) Regarding bridge rectifier before opto,I wii have provision on my developement board to have a choice by 2 jumpers because of the idea in the Elektor article mentioned above.Afer all without full wave my code gets some breathing time.Actually I also thought thus:
Whenever the software is not busy processing e received RC5 code,[or polling switces],it is constantly measuring the period duration of the mains voltage.The measured value is divided by two to enable the software know when the triac is to be fired during the missing half wave.The zero crossing detector employs Ext0 interrupt trigered by a falling edge.When sich an interrupt is signalled,Timer1 is started.When an overflow occurs,a Timer1 interrupt is generated which leads to the Triac being fired.Next,Timer1 is set to previously measured value of the half period length and restarted.
After 10ms(50Hz) the second fire pulse occurs.A flag must be used to know whether Timer1 is loaded with 10ms or desired delay.
(c) Can you share the mains filtering circuit?
(d) I do not know whether I understand state machine.But still I have written a LCD chess clock of FIDE approved standard(I am a certified chess arbiter)which means there are a lot in the menu:many fixed equal time intervals as also selectable (three keys) individual time intervals(0 to max) plus Fischer Module (again selectable time gain per move) etc.I think it was in context of menu that Michael Karas published his 8 led state machine asm code.I tried that with success but without really understanding.Then after a long time it was you who explained that code but again dummy like me could not fully understand.Probably it was out of context hence I could not give enough time.Or may be real teaching need examples. Of course,NOW,if you please care to give some example in ASSEMBLY,FOR RC/5 state machine for short low, short high, long low and long high that will benefit many not only me.I shall be gratefull.
With best regards
A.S.Rudra