# LED PWM Frequency design for two wheeler turn indicator

#### hoyyoth

Joined Mar 21, 2020
307
Dear Team,

I need to design a LED turn indicator for two wheeler.
The standard requirement is given below.

I need to calculate the blinking frequency from the above spec.
Does that means my pwm frequency is[ (90+/-30)/60]Hz.

I will be using BD18327EFV-M for this application.

Regards
HARI

Last edited:

#### Ya’akov

Joined Jan 27, 2019
8,973
The PWM control of that IC is not about blink rate. It is used to reduce the current to the LEDs.

Look at the section 3 of the datasheet on the CR Timer:

“Datasheet” said:
This IC determines the flasher cycle from the internal clock generated by CR timer. The CR timer period, ON Duty, can be
set by the external resistor RCRT1, RCRT2 and the capacitance CCRT.
This is where you set the blink rate. You can either ignore the PWM (in which case you get 100% duty cycle) or choose to use it to reduce current to the LEDs.

Considering what this IC does, it is very complicated. I think they should have simplified the application by abstracting some of this rather than making it so directly coupled to the architecture.

#### hoyyoth

Joined Mar 21, 2020
307
Hi,
Thank you very much.
May I know my led blinking frequency calculation is correct or not.

Led blinking frequency is[ (90+/-30)/60]Hz.

Regards

#### Jerry-Hat-Trick

Joined Aug 31, 2022
494
Given the relatively high unit price and complexity as well as having to use a surface mount device with so many pins I’d seriously consider using a 555 astable, even two separate circuits for left and right for simplicity and cost reasons.

#### hoyyoth

Joined Mar 21, 2020
307
Hi
In the datasheet page no 22 you can see an application example.
I am tonally confused about it.

In the application example the clock frequency is 763Hz.

1)May I know this 763Hz is derived from LED blinking frequency of 1.49Hz.

2)As designer the Input I have will LED blinking frequency.I can't see any equations which relate LED blinking frequency and CR timer.

3)From the LED blinking frequency how to decide how long led will be on and off

I am keeping the datasheet link and application circuit again here for reference.

#### LowQCab

Joined Nov 6, 2012
3,937
It would be much simpler to just use a standard Automotive-Turn-Signal-Flasher, and cheaper too.
The type that You need has 3-Terminals, not 2-Terminals.
You can get one for less than ~\$5.oo.
.
.
.

#### hoyyoth

Joined Mar 21, 2020
307
May I know by " Automotive-Turn-Signal-Flasher " you mean LED blink Flasher Relay.

#### LowQCab

Joined Nov 6, 2012
3,937
The "3-Terminal-type" of Flasher will flash almost any Load attached to it.
.
.
.

#### eetech00

Joined Jun 8, 2013
3,823
Dear Team,

I need to design a LED turn indicator for two wheeler.
The standard requirement is given below.

View attachment 308792

I need to calculate the blinking frequency from the above spec.
Does that means my pwm frequency is[ (90+/-30)/60]Hz.

I will be using BD18327EFV-M for this application.

Regards
HARI
f=60/90=0.666667 Hz
p=1/f=1.5 sec

PWM is related to duty cycle.
50% duty is roughly p x 0.5=0.75 sec

What type of two wheeler?

#### hoyyoth

Joined Mar 21, 2020
307
Thank you very much.
This is an electric bike.
In the datasheet application example how they arrived the clock frequency 763Hz

#### eetech00

Joined Jun 8, 2013
3,823
The frequency was probably chosen to meet an automotive lighting standard.
You'll have to do some research to find out why 736Hz was chosen.

#### hoyyoth

Joined Mar 21, 2020
307
automotive standard is

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