Problem with interfacing relay with a microcontroller

Am using a microcontroller to switch a relay . I have made sure the transistor gets enough bias current to switch the relay . A flywheel diode is properly connected across the relay .The microcontroller is programmed in such a way that it triggers a relay after 20 seconds of receiving an input signal . I am using an assembly language and used a " set bit " command to trigger the relay .
My problem is , after receiving the signal for 20 seconds , the relay switches and stays on only for 2 seconds . this relay is supposed to be on infinitely until the input signal is removed .

What do I do to make it stay

Hi Chris,

Have you turn off the timer after being trigger? In some MCUs there is a watchdog timer that is sometimes turned off to continue the operation.

As I understand your case, there is an insufficient supply to your relay.

God Bless.

Best,
Neil

Hi , Christ,
1 check your power supply filters and supply voltage .
2 give the supply direct to the relay coil and see that it holds the relay for your required
time if not connect a 100 mfd 63v cap across the rly coil.or increase the supply voltage for proper holding of the relay the cap avoids rly chattering.
3 check the base to emitter voltage of the relay switching transistor it should not be
less than 0.7v dc.If it is a darlington tr the will be still higher.

4 another way to hold the rly permanent is by using a dual contact rly use one contact for ur switching purpose and use the no/common of the other contact of rly to give permanent supply to the rly switching transistor make sure you provide a re set switch series to the common point of the rly the switch should be a push to off switch. once the rly holds it will not release until the re set switch is activated.


Regards, Sambath kumar