single power supply + communictaion

[amitaiwe]

Hi everyone,
I've build a metal detector circuit using a 555 timer as explained in this link:
**broken link removed** al-detector-using-555-timer.html
instead of using a speaker as the informer of detecting I am checking the frequency at the point were the speaker is shown in the diagram. The frequency is measured by a capture module of a uC (pic16f877) which captures the value of a timer (which is zeroed before capturing) every rising edge of the oscillator created by the 555. The data at the uC is been compared to the last value been measured and then summed up. After a few cycles the data of the summed information is been transmitted to a BT module (via UART protocol) and from there to a PC.
For checking that the uC works well I have added in the programming a led flickering section in the main cycle of the system.
When the led isn't flickering that's an indication the the system isn't functioning properly. for checking that the transmission works properly I have programmed the uC so to send before the main cycle a 5 4 3 2 1 packet.
when the 555 circuit is disconnected and the capture module of the uC is connected to the external crystal oscillator (as a demo check) the data received at the PC's terminal functions properly ( 5 - 1 and then a fixed sequence).
When I try to connect the 555's circuit to the uC circuit and using the same voltage supply for the different components (BT module, uC, 555 timer) the data received at the PC's terminal is very strange:
Or that its a sequence of 5-1 5-1 .... or that it's a sequence of FF to FB ( -5 to -1 in HEX).
My guess is that the problem for the disruption in the communication is due to the fact of not using multiple supplies or using a LDO. I have read about the issue of using one supply for multiple components and as far as I understand the simple effect of doing that will be that one of the components won't get the current it needs and therefor the system won't function, but why using the the same supply for the uC and the 555 causes for changes in the data?
The power supply for the circuit is a DC power supply of 5v (from the socket) and I have in my disposable a few 9V
batteries. Should I try using for every component a different battery with an LDO? (I am aware of the face of it being a great power-loss supply) or maybe to look in a different direction?
Thanks, Amitai

 

[erikl]

Did you block the power supplies of the chips? You should add a ceramic capacitor of about 0.1µF or/and a 1..10µF (Tantal) electrolytic capacitor as close to the power supply pins as possible - at each chip, at the BT module, too.

 

[amitaiwe]

Hi again,
I have done a few experiments and this is my current system schematic:


What is supposed to be in the system is:
1.Oscillating a frequency based on the 555 timer circuit which includes the RLC part and all of this together is included
in the sensor part. This circuit is fine by it self and it's frequency is fixed at 465HZ on steady state.When a metal object is close to the inductor the frequency gets to 411HZ and is fixed too.
2.a PIC16f877 capture module is sampling the frequency and then transmitting the result to the Bluetooth module,
whom passes over the data to a PC terminal.

I managed twice during the experiments to implement the whole system with good result, i.e. the values received in the terminal (which are proportional to the frequency's value) clearly change
when a metal object is close to the inductor.
​​
for some reason this situation isn't stable enough and I'm not sure why. I have tried doing several things(Chronological):

- For the sake of checking if there's any disturbances between any of the systems components I connected them gradually to each other. first the 555 oscillator, which as I mentioned is fine. When I tried to connect to the same power supply the BT and PIC the frequency became unstable.

- Therefor I tried to feed the power supply's signle through a L7805 regulator. As result the circuit didn't get enough
energy so for all the components to operate (led-on indicators weren't on). So I increased the voltage input of the regulator from 5V to 7V which led the Vout of the regulator to be ~5.4V.
That's when the system results where good ones.
after fifteen minutes​​ i noticed that the frequency is again unstable and the regulator is very hot.


- Testing the input currents of the 555, uC and the BT results in maximum a few tens of mA.

- I then added another power source as seen in the schematic - The oscillator circuit has a 5V power source and the BT+uC has a 5V power source. I do not know if this is a good solution but as something temporary such separation doesn't cause the collision between the two parts of the system and the frequency is not interrupted.

- Trying again to sample the timer's frequency when some metal object is close to the inductor didn't give any result in the terminal software (The 555 oscillator is fine and is accurate while testing with a DMM), although Seemingly this is the same circuit which was in good shape for a while. The only change I can think of from the original is that the input voltage (single supply) had a bigger value( 7V ) but this voltage level is also what caused the regulator to overheat (maybe?).

In the schematic can be seen a bi-colored LED that flickers every operation cycle, so that if the uC isn't really sampling a frequency the continues loop operation stops. Therefor the LED is an indication of sampling progress. In retrospect, it seems the output of the sampling leg of the uC is very sensitive to changes so that even when connected to a constant voltage node it is also blinks sometimes. For examination purpose I sampled the oscillator's crystal oscillator and this result in a very rapid flashing of led, as compered to a situation of connecting to a constant voltage node. So it seems that there's a real frequency sampling process which might be influenced in a high rate from unwanted noise signals.

I would be glad for help trying to achieve a good steady state situation for the system,
Thanks
Amitai