help regarding simple comparator

Audioguru

Joined Dec 20, 2007
11,248
The collector of the output transistor goes low while the 555 is timing.
The emitter of the output tranistor goes high when the 555 is timing.

Did you apply 6V to the base of the input transistor when its emitter is 0V without a series current-limiting resistor? Then it is blown up.

With a 6V supply, the output-high voltage of the 555 is about +4.6V and the emitter of the output transistor is from +3.6V to +3.8V. So of course a 6V motor will not be at full speed.
 

Thread Starter

devalvyas

Joined Nov 11, 2008
80
Aguru, i get your point, i will apply the 470ohm current control resistor to input transistor. Also in the input trigger curcuit that Bill showed, i am putting only only two resistors after the input transistor, i.e. R3 and R1. I am not using R2. will it affect ?

also, bill, pl give your ideas on how to increase the motor speed? As A guru said, the emitter of output transistor is 3.8 volts only.

I am considering the output of the 555 to be connected to a pwm ic to get more speed and to control the speed of the motor.... I have not yet tried it, but will be trying it tomorrow....can it be done?
 

Wendy

Joined Mar 24, 2008
23,415
The fundimental problem is the 555 output is not capable of going all the way to the plus rail. Like I said, there are ways around this, I worked OT this weekend, which always wipes me out.

PWM will work, but it will only slow it down from the max you now have. Let me tweak the existing schematic and get back with you. Be back shortly.

**************************



I've eliminated D1, since it wasn't doing anything.

C6 is the important part of making it work, did you view the page I pointed you to? R5 will protect the transistor from surges, but is less important.

You do need R2. See this page.

Common emitter will work, but it has to be off for the motor to be able to generate a signal. The problem with the 555 is it is switched either on or off, hard. There is a transistor connecting the output to the power supply either way, with this scheme there is only a transistor connected when the motor is running.
 

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Thread Starter

devalvyas

Joined Nov 11, 2008
80
Dear Bill, I tried the thrid circuit but its not working...i am not getting pulse in input transistor Q2 even though i am moving the motor...may be i am wrong, i will try again..

i have posted the exact schemetics of how the motor is currently connected to generate the trigger as well as to run. File 555 and trigger ckt.JPG

When DCMotor (excuse my symbol, i dont have motor symbol in pspice that i use) runs in clock wise direction, it generates a pulse, which goes through cap c4 and triggers input-trigger-transistor q2. This pulse is further conditioned by trigger circuit R6-C3 and inturn triggers 555. The output of 555 triggers Output Transistor Q1. The time of pulse is determined by timing circuit C1-R4.

The DCMotor is also connected to emitter of Q1. Hence when Q1 is on, the motor starts running in the same direction as initially moved.

Now correct me if i am wrong:

The moment Q1 is on the entire 6 Volts (minus the drop in Q1) is applied to the motor. since the motor is in common emmiter config of Q1, the current gain is max and hence the common collector config was not working earlier. Also, 555 is only involved in triggering Q1 and the DCMotor actually runs through Q1, hence there should not be substantial drop in the voltage applied to DCMotor to run.

In the config that Bill has shown, i am not clear how to connect the DCMotor back to Q1. Shall i connect the emitter to the +ve terminal of the DCMotor? If i do that, how will the motor run without the ground? Also, when the motor is running in reverse direction, the Diode D4 is short and the entire negative pulse is applied to base of Q2. Won't this destroy Q2?

Now the problem:

1. I have to really move the motor hard to generate the required pulse for input transistor Q2. How do i make the circuit more sensitive to movement, so that little movement in clockwise direction, which generates some 0.5 volts, should trigger Q2.

2. Also the motor is still not running on full speed as i thaught. I might be wrong in assuming that motor is actually getting power only through Q1 and 555 has no role in voltage drop except for triggering and timing.

Again, thank a lot for your feedback guys….





The fundimental problem is the 555 output is not capable of going all the way to the plus rail. Like I said, there are ways around this, I worked OT this weekend, which always wipes me out.

PWM will work, but it will only slow it down from the max you now have. Let me tweak the existing schematic and get back with you. Be back shortly.

**************************



I've eliminated D1, since it wasn't doing anything.

C6 is the important part of making it work, did you view the page I pointed you to? R5 will protect the transistor from surges, but is less important.

You do need R2. See this page.

Common emitter will work, but it has to be off for the motor to be able to generate a signal. The problem with the 555 is it is switched either on or off, hard. There is a transistor connecting the output to the power supply either way, with this scheme there is only a transistor connected when the motor is running.
 

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Thread Starter

devalvyas

Joined Nov 11, 2008
80
I tried replacing the output transistor Q1 with a 6 Volts Relay. The out put of 555 goes directly to the relay. The relay acts as a switch between the 6v battery and the motor. When the output of 555 goes high the relay closes and i get the full voltage of the battery for the motor..

however when the out put of the 555 tries to go low, the relay is not opeaning up and there is sputtering inside...

how to overcome that?
 

Wendy

Joined Mar 24, 2008
23,415
Keep the transistor. Have you tried reversing the motor leads? You'll note the diode has reversed also, even if it looks the same. Look at the side connected to the input of the circuit. You should be able to trigger the circuit like you did before without any feedback.
 

Thread Starter

devalvyas

Joined Nov 11, 2008
80
Sorry for delay in responce....can you please let me know how i can get the max voltage at the motor terminals? I am using a 6volt relay at output of the 555 timer...the relay closes well, however it does not open up properly and there is a sputtering noise inside the relay..how do i do this properly....

I tried replacing the output transistor Q1 with a 6 Volts Relay. The out put of 555 goes directly to the relay. The relay acts as a switch between the 6v battery and the motor. When the output of 555 goes high the relay closes and i get the full voltage of the battery for the motor..

however when the out put of the 555 tries to go low, the relay is not opeaning up and there is sputtering inside...

how to overcome that?
 
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